
Glass 



^v^ 




PHILOSOPHICAL C/ 

CONVERSATIONS: ' 



IN WHICH 



ARE FAMILIARLY EXPLAINED 

THE CAUSES OF MANY DAILY OCCURRING 
NATURAL PHENOMENA. 

BF 

FREDERICK C. BAKEWELL. 



WITH NOTES, AND QUESTIONS FOR REVIEW \ 

BY ^ 

EBENEZER BAILEY, 

Principal of the Young Ladies' High School, Boston 3 Author of « First Lessons 
in Algebra," " Young Ladies' Class Book," &c. 




tereotgpe 25&ftfon 




BOSTON: 
CARTER, HENDEE & CO. 



1834. 



7«\ V 



- \ 



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Entered according to Act of Congress, in the year 1834 

By Carter, Hendee & Co. 

In the Clerk's Office of the District Court of Massachusetts. 



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PREFACE 



The object of the present work is to explain the 
causes of those phenomena which either pass unre- 
garded, from the frequency of their occurrence, or 
are considered too abstruse to be comprehended with- 
out a previous acquaintance with the elements of 
science. It has been the Author's endeavor to ac- 
complish this object in the most clear and intelligible 
manner, and to enter so fully into the nature of the 
subjects illustrated, as to convey a competent knowl- 
edge of the principles on which their effects depend. 
The conversational style has been preferred, as it is 
not only the best adapted to keep up the attention of 
young people, but it also affords opportunities for the 
suggestion and solution of such difficulties as present 
themselves, and renders the explanations more readily 
understood, and more strongly impressed upon the 
memory. 

The developement of the causes of the phenomena 
elucidated in this volume does not depend exclusively 
upon any one branch of science. The subjects select- 
ed for illustration are those which were considered 



4 PREFACE. 

most likely to come within the range of daily obser- 
vation, and to be best calculated to rouse the atten- 
tion, and to excite a taste for scientific inquiry. 

In some of the explanations, the Author has ven- 
tured to deviate from the opinions generally received ; 
but he has never done so, excepting when the causes 
usually assigned appeared to be either very doubtful 
or unsatisfactory. The Conversations on Clouds and 
Fogs, Rain, the Barometer, Winds, and Vision, are 
those to which this notice more particularly applies. 

Hampslead, Jan. 1833. 



NOTE 



BY THE AMERICAN EDITOR. 



The Editor does not offer this little work to the American 
public, as a full and systematical treatise upon the subject of 
Natural Philosophy. It is a book especially designed for the 
use of beginners ; and the author has explained many of the 
common and interesting phenomena of nature in a manner so 
familiar and simple, that even children can hardly fail to under- 
stand their causes. In this way, if they are not made "philoso- 
phers, they will, at least, become acquainted with some of the 
most important truths and useful principles of philosophy ; and, 
which is of vastly greater importance, they will learn to think, 
and will acquire the habit of investigating the causes of things. 

These " Conversations" make no pretensions to the scientific 
arrangement, usually found in books of philosophy; nor do 
they profess to explain all of even the most remarkable phe- 
nomena of nature. As the author would excite the attention 
and awaken the curiosity of the young, and thus promote a 
taste for scientific investigation, he has judiciously selected 
such subjects as are the most interesting, and avoided the 
tediousness of an artificial arrangement. The principles he has 
undertaken to illustrate, are such as are suggested by almost 
daily observation. 

The Editor has omitted a few paragraphs, which he thought 
useless in an American edition of the work ; and he has made 
1* 



6 NOTE BY THE AMERICAN EDITOR. 

some verbal alterations, which he believed the author himself 
might make in a second edition. 

The Questions annexed to the several Conversations, are in- 
tended rather for the use of the scholar than the teacher. 
After a Conversation has been carefully studied, — not committed 
to memory, but thoughtfully read over, — the learner should 
examine the questions ; but not before. If he can answer them 
all, he may be satisfied that he understands his lesson. If he 
cannot, let him read over the Conversation again ; but in no 
case should he try to find answers to the questions, with the 
view of committing them to memory. Let him commit the 
ideas of the author to memory, and understand them ; but he 
should explain them in his own words. The order of the book 
is not always followed in the questions; and questions are 
frequently proposed, to which no answers are given. But the 
ingenious scholar will be able to answer all such, if he 
thoroughly understands the Conversation to which they refer. 

E. B. 
Boston, My, 1833. 



CONTENTS 



Introductory Conversation. — Diffusion of Heat. 
Metals better Conductors of Heat than Wood — Use of wooden Han- 
dles, — Experiment. — Metals sooner cooled than bad Conductors. 
— Cause of Money in the Pocket feeling hot. — Water a bad Con- 
ductor of Heat, — Experiment. — Mode by which Fluids impart and 
absorb Heat. — Radiation of Heat page 13 

II. — Warmth of Clothing. 
Metals really as warm as Woollens, — Experiment. — Difficulty of 
distinguishing Heat from Cold by the Touch, — Experiments.— 
Cause of the sensible Warmth of Woollens and Furs 22 

III. — Cold of Wind. 
Wind not really colder than a Calm, — Experiment. — Cause of Air 
in Motion feeling cold, and cooling heated Bodies, — Experiment. 
— Effect of Clothing in protecting the Body from Change of Air. 
Hot Winds, — their Effects, — Experiment. — Fans, — the Air heated 
by their Motion 29 

IV.— Cold. 

The Sensation of Cold produced by partial Abstraction of Heat. — 
Heat in Snow,— Experiment. — Effects of very low Temperatures. 
— The Non-existence of Cold as a positive Property 35 

V. — Expansion by Heat. 
Expansion of Mercury by Heat — The Thermometer, — Experiment. 



8 CONTENTS. 

— Specific Gravity diminished by Heat. — Expansion of Air. — Fire 
Balloons, — Experiment. — Draughts up Chimneys. — Causes of 
their smoking. — Spiral Serpent. — Smoke Jack. — Expansion of 
Metals. — Expansion of Water, — Peculiarity attending it; — Import- 
ance of its Deviation from the general Laws of Expansion by 
Heat page 41 

VI. — Boiling. 

Water made to boil by Cold, — Experiment. — Cause of Boiling. — 

Effect of the Pressure of the Atmosphere on the Boiling of Fluids, 

— Experiment. — Heat carried off by Steam and Vapor. — Sensation 

of Cold from Wet Clothes 52 

VII. — Steam and Vapor. 
Steam invisible. — Condensation of Steam by Cold. — Vapor of Spirits. 
— Great Bulk of Steam. — Vacuum caused by Condensation, — Ex- 
periment. — Condensing Steam Engines. — Elastic Force of Steam, 
— Experiment. — Accidents from the Bursting of Steam Boilers. — 
High Pressure Steam-Engines, — their Advantages. — Heat of 
Steam, — Experimen 60 

VIII. — Clouds, Fogs, and Dew. 
Evaporation. — Clouds composed of condensed Vapor, — how sup- 
ported in the Air. — Fogs. — Difference between Fogs and Clouds. 
— Formation of Dew ; — the Quantity formed, different on different 
Substances; — Cause of this Difference. — Damp Walls in a Thaw. 
— Breath visible in a Frost 72 

IX. — Rain, Snow, and Hail. 

Clouds changed into Mists. — Different Densities of Clouds. — En- 
largement of the Drops of Rain as they descend. — Shower of 
Rain from condensed Steam. — Effect of Wind in producing Rain. 
— Formation of Snow and Hail 80 



CONTENTS. 9 

X.— Fire. 
The Air the Cause of Fires burning, — Experiment .—Latent Heat, 
— Experiment. — Latent Heat of Water, Steam and Air. — Water 
produced by Fire page 88 

XL — Fire (continued.) 
Composition of the Air. — Oxygen, Nitrogen, and Hydrogen Gases, 
— Experiment. — Combination of Oxygen and Hydrogen Gases in 
Water, and their Separation. — Hydrogen in Coals and Tallow. — 
Red Heat. — Caibon and Carbonic Acid Gas. — Flame. — Smoke. — 
Combustibles not destroyed by Burning. — Indestructibility of Mat- 
ter 95 

XII. — The Barometer. 
The Pressure of the Atmosphere. — Variations in the Pressure. — 
Connection between the Pressure and the Weight of the Air. — 
Construction of the Barometer, — Experiments. — Measuring 
Heights. — Atmospheric Pressure not dependent on the Specific 
Gravity of the Air. — Changes in the Height of the Atmosphere. — 
Wind the Cause of Variation in Atmospheric Pressure. — Effect 
of Motion in diminishing Pressure, — Experiment. — Atmospheric 
Pressure on the Human Body. — Application of the Barometer as 
a Weather-Glass " 103 

XIII.— Winds. 

Effect of Heat in causing Winds. — Causes of their Variableness. — 

Trade Winds, Monsoons, and Land and Sea Breezes. — Theories 

of the Trade Winds. — Tides of the Atmosphere. — The Velocity 

of Wind,— its Power 116 

XIV. — Light. 
The Nature of Light, — its Velocity. — Rays of Light invisible, — 
Experiment. — Reflection and Diffusion of Light from Mirrors, 



10 CONTENTS. 

Crystals, and rough Surfaces. — Diffusion of Light by the Vapors 
of the Atmosphere and Clouds. — Twilight page 128 

XV. — Light (continued.) 
Darkness, — Objects visible in the Dark. — Shadows. — The Decrease 
of Light by Distance. — Distance to which Light extends. — Light 
from the Moon, — how produced, — its Quantity 138 

XVI. — Refraction of Light. 

The Refraction of Light in passing from one Medium into another, 
— Experiment. — Important Uses of Reflection. — Refraction of 
Light by the Atmosphere, — its Effects on the Appearance of the 
Stars, — Experiment. — Apparent and real Depths of Rivers. . . 147 

XVII.— Colors. 
Separation of the Rays of Light by the Prism, — Experiment. — Dif- 
ferent Refrangibility of the colored Rays. — Colors of Objects. — 
The Rainbow. — Haloes. — Black, the Absorption of Light. — De- 
ceptions produced by Colors, — a Red Mourner, and a Walking 
Skeleton 154 

XVIII. — Reflection. 

Objects seen in Looking- Glasses. — Laws of Reflection. — Angles of 
Incidence and of Reflection, — Experiment. — The Fire seen out- 
side the Window. — Objects apparently behind Plane Mirrors. 164 

XIX.— Vision. 

Composition of reflected Light. — Effect of bringing the Rays to a 
Focus, — Experiment. — The Eye. — Images of Objects inverted on 
the Retina, — how seen upright, — Experiment. — The inverted 
Theory of seeing Objects upright refuted. — Near-Sight and Long- 
Sight, — how corrected by Spectacles, — Experiment. — Mechanism 
of the Eye 171 



CONTENTS. 11 

XX. — Vision (continued.) 
Apparent Size of Objects, — their smallest visible Magnitude, — their 
Sizes when distant, how estimated. — Comparison of Objects. — Il- 
lusion produced by unusual Contrasts. — Perspective. — Apparent 
Motion of distant Objects. — Duration of Impressions. — Single 
Vision with two Eyes. — Enlargement and Contraction of the Pupil. 
— Eye of the Cat. — Seeing Objects in the Dark page 182 

XXI. — Magnifying Glasses. 

Effects of Convex Lenses. — Objects not really magnified by them. — 

Natural Size of Objects. — Refraction of Light in passing through 

Plano-convex and Double-convex Lenses. — Convergence of the 

Rays of Light to a Focus 191 

XXII. — Images of Objects. 
Burning- Glasses, — the Image of the Sun formed by them, — their 
Effects. — Size of Images formed by Convex Lenses, — Experi- 
ments. — Why the Images are inverted. — Size of the Images pro- 
portioned to the focal Length of the Lens. — Application to the 
Measurement of Heights. — Images formed on the Retina of the 
Eye 198 

XXIII. — Optical Instruments. 
Telescopes, — their Magnifying Power, — why limited. — Compound 
Microscope. — Solar Microscope. — Magic Lantern. — Phantasma- 
goria 206 

XXIV. — Concave and Convex Mirrors. 
Images formed before a Concave Mirror. — Difference in their Sizes. 
— Why inverted. — Magnifying Power of Concave Mirrors. — Re- 
flection of the Rays of Light from Convex Mirrors. — The Cause 
of Objects appearing less than when reflected from Plane Mir- 
rors 216 



12 CONTENTS. 

XXV.— Kites. 

An oblique Position essential to the Ascent of Kites.— The ascend-! 
ing Power produced by the Reflection of the Wind.— The three 
Forces acting upon Kites.— Use of the Tail and Bobs,— Expert- 
ments.— Effect of the Sides of the Kite being inclined backwards, 
-Experiment page 224 

XXVI.— Sailing. 

Sailing against the Wind,— Experiment.— The Rudder,— its Action 
in turning Ships.— The impelling Direction of the Wind changed 
by Reflection from the Sails.— Resistance of the Water— Oblique 
Course of Ships.— Tacking.— The united Action of the Rudder, 
the Sails and the Water 234 

XXVII.— Flying. 
Action of the Wings of Birds upon the Air.— Birds flying horizon- 
tally and upwards without moving their Wings.— The Ascent of 
Birds by the Impulse of their Weight.— Use of the Tails of Birds. 
—Power of Ascent gained by the Resistance of the Air.— The 
Soaring of Kites. — Practicability of Men flying. — Impracticability 
of guiding Balloons.— Means by which Man may expect to fly.— 
Conclusion. m 242 



PHILOSOPHICAL 
CONVERSATIONS 



INTRODUCTORY CONVERSATION 

DIFFUSION OF HEAT. 



A Parlor in Mr. Powell's House, in which Mrs. 
Powell is making Tea, with Mr. Powell and their 
Three Children, Frederick, Robert, and Harriet, 
seated round the Table. 

Mr. Powell. — Can any of you, my children, tell me 
why the handle of the teapot your mamma is using, is 
made of wood ? 

Harriet. — I suppose it is to prevent the hand from 
slipping, as it would on a polished silver handle. 

Mr. P. — No, that is not the principal reason. The 
handle is made of wood to prevent it from becoming too 
hot to hold, as it would if made of metal. 

Harriet. — Why should a metallic handle become hotter 
than a wooden one ? 

Mr. P. — Because metal is a better conductor of heat 
than wood ; and the heat from the boiling water in the 
teapot would, therefore, be sooner conveyed to it. 
2 



14 DIFFUSION OF HEAT. 

Mrs. P. — You may quickly convince yourself of that, 
Harriet, by touching the silver at the point where it joins 
with the wood. If, in making tea, you allow your finger 
to slip off the wooden handle, you will soon feel the metal 
burn you. 

Frederick. — Is it for the same reason that people use 
paper and woollens to take hold of the kettle or hot irons ? 

Mr. P. — Yes. Paper and woollens are, comparatively, 
bad conductors of heat ; therefore it is longer in penetrat- 
ing them. 

Robert. — But, I suppose, when the heat has once got 
through them, they would be as hot as metals. 

Mr. P. — They would be as hot, but they would not feel 
nearly so hot, as metals of the same temperature. The 
property metals possess of becoming quickly heated, when 
brought near to a heated body, also disposes them to part 
with their heat quickly, when touching a colder substance. 
Both effects are produced by the facility with which metals 
conduct heat. 

Robert. — But I cannot understand how the same cause 
that makes metals sooner hot than other things, should 
make them cool sooner also. 

Mr. P. — I will endeavor to explain the difficulty. 
When any cold substance touches a heated body, it takes 
away a portion of the heat from the part it touches ; and, 
if the heated body be a bad conductor, it will require 
more time before that part which is deprived of heat can 
be again supplied with it from the other parts of the heated 
body. In this manner the heat will be retained longer in 
the bad conductor. In metals, on the contrary, the heat 
is quickly conveyed from every part of the metal to any 
substance touching them; consequently they become 
sooner cold. Thus, you perceive, a heated piece of metal 
must feel hotter than a heated bad conductor, because, 



DIFFUSION OF HEAT. 15 

though both may be equally hot, the metal gives out its 
heat more rapidly. 

Frederick. — I see now what you mean, father ; heated 
metals feel hotter than woollens, because they give out a 
greater quantity of heat in the same time. 

Mrs. P. — This will also explain to you, I think, the 
cause of the money in your pockets feeling so hot, at 
which you were puzzled so much the other day, when 
standing by the fire. 

Mr. P. — Exactly so. 

Robert. — But how can we tell that the metal is not the 
hottest, after all? 

Mr. P. — By means of the thermometer* we may ascer- 
tain their degrees of heat to be the same. It is true, if 
you were to surround the bulb of the thermometer with 
heated metal, the mercury in the tube would rise more 
rapidly than when covered with heated woollen ; but, in a 
short time, the woollen would raise the mercury as high as 
the heated metal. I can convince you that metals con- 
duct heat more quickly than wood, by a very simple 
experiment. 

Robert. — I should like to see it, father. 

Mr. P. — This silver spoon and this wooden one are of 
the same length, you perceive : hold one in each hand at 
the farthest end, and dip them both into this basin of 
boiling water. (Robert does as his father desires ; but, 
after holding the spoons about a minute, he lets the silver 
one fall into the basin, and draws his hand quickly away.) 
What is the matter, Robert ? — you shake your hand as if 
the silver spoon burnt you. 

* The principles upon which the thermometer is constructed, will be ex- 
plained in a subsequent conversation. The manner in which it indicates 
different degrees of temperature, should be exhibited to the pupil in this 
place. — Am. Ed. 



16 DIFFUSION OF HEAT. 

Robert. — And so it did. It is so hot I could not hold 
it any longer. 

Mr. P. — How does the wooden spoon feel ? 

Robert. — Scarcely warm where I have hold of it; 
though, lower down, it is quite hot. 

Mrs. P. — I think you must be satisfied now, Robert, 
that silver conducts heat more quickly than wood, as you 
have burnt your fingers in the trial. 

Robert. — Yes, it must be so : but do all metals con- 
duct heat as quickly as silver? 

Mr. P. — No, they vary very considerably. It has been 
found, by recent experiments, that gold is the best con- 
ductor ; next to gold is silver ; then copper, iron, and tin. 
Lead possesses this property in a much lower degree. 

Harriet. — If the heat of the water made the silver 
spoon so soon hot, I suppose water is a good conductor of 
heat also? 

Mr. P. — You are mistaken there, Harriet ; for water is 
not only a very bad conductor of heat, but it has been 
supposed to be not capable of conducting heat at all.* 

Harriet. — Then how could it part with its heat so 
rapidly to the spoon? 

Mr. P. — The particles of the water were put in motion 
by Robert, when he plunged the spoons into the basin ; 
therefore a fresh portion of the heated water was every 
instant made to touch the spoons, and the silver one quickly 
took the heat from the different particles as they touched 
it. Had the water been in a state of perfect rest, the spoon 
would not have been heated nearly so soon ; because the 
heat, taken away from those particles immediately sur- 

* It is true that water is a very bad conductor of heat j but satisfactory 
experiments have demonstrated that it is not a perfect non-conductor. 
Heat has been propagated downwards through water contained in a vessel 
formed of ice. — Am. Ed. 



DIFFUSION OF HEAT. 17 

rounding the spoon, could not have been renewed by the 
heat in the other part of the water, owing to its non-con- 
ducting property. 

Frederick. — Is the heat in liquids communicated, then, 
by their motion, rather than by their power of conducting 
heat? 

Mr. P. — That is generally the case, Frederick ; and 
while their particles keep in motion, and are thereby pre- 
senting a constantly changing surface to the body touching 
them, the heat will be supplied as fast as it can be con- 
ducted away. 

Robert. — Can you show us that this is the case by any 
experiment ? 

Mr. P. — There are several experiments by which I 
could convince you that water, when not in motion, is a 
very bad conductor. I will show you one now. 

Harriet. — Do, dear papa, for I delight in experiments, 
they seem to make things so clear. 

Mr. P. — I will half fill this ale-glass with cold water, 
and put the bulb of the thermometer into it, that we may 
see if any change takes place when I pour hot water upon 
the cold. You observe, the thermometer in the glass is 
now at 50°. I place this piece of paper on the top of the 
cold water, while I pour the hot water gently upon it, to 
prevent their mixing. 

Harriet. — The hot water you are pouring in is of a 
different color from the cold. 

Mr. P. — I have put a little ink into it, to enable us to 
distinguish the two waters. — You see I have poured the 
water so gently that they have not mixed. I will now 
remove the small piece of paper that divides the hot from 
the cold water, and if I do so carefully, they will remain 
nearly as distinct as they are now. (Mr. Powell removes 
2* 



18 DIFFUSION OF HEAT. 

the paper.) You see the hot water remains in a separate 
layer at the top. 

Harriet. — How very curious it is that they do not 
mix! 

Frederick. — The thermometer has risen only three 
degrees, though the colored water must be nearly boiling 
hot, and is within two inches of the bulb. 

Mr. P. — Nor will it rise much higher if it remain in the 
water half an hour. I hope this experiment convinces 
you, Robert, that water, when at rest, is a very bad con- 
ductor of heat.* Indeed, most fluids, with the exception 
of quicksilver, are very imperfect conductors, and impart 
heat to, and abstract it from, surrounding bodies, princi- 
pally by the agitation of their particles, in consequence of 
which a fresh surface is constantly brought into contact 
with the body they surround. It is in this manner that air, 
which is a bad conductor, becomes capable of communi- 
cating and taking away heat very rapidly. 

Harriet. — But, papa, I have noticed that, as soon as I 
come in sight of a large fire, I feel a glow upon my face : 
is it owing to the heat being conducted by the air in that 
way? 

Mr. P. — The effect you have observed is produced by 
another most interesting property of heat, called radiation. 



* The following experiment proves this fact in a very striking manner. 
If a cake of ice be confined in the bottom of a glass tube, filled with water, 
and the tube be then held obliquely over a lamp, in such a way that only the 
upper part of it shall become heated, the water on the surface may be made 
to boil, while the ice remains unmelted below. If, instead of ice, the bottom 
of the tube contain a small portion of colored water, the two strata will 
remain distinct, when the heat of the lamp is applied at the top of the tube 5 
but if it be applied at the bottom, the colored water will immediately rise, as 
it becomes rarefied, and equally diffuse itself through the whole mass. 
— Am. Ed. 



DIFFUSION OF HEAT. 19 

Frederick. — What do you mean, father, by the radiation 
of heat ? 

Mr. P. — It has been discovered that all bodies, besides 
communicating heat to substances touching them, have 
the power of emitting heat from their surfaces as rapidly 
as light. As the heat thus emitted proceeds from bodies 
in straight lines, in all directions round them, like the radii 
from the centre of a circle,* it is hence called radiant, or 
radiated, heat, to distinguish it from conducted heat. The 
quantity of heat radiated from different bodies depends 
more upon their surfaces than upon their internal qualities. 
Highly-polished metals radiate the least heat, and dark 
substances the most.f 

Mrs. P. — Is not that the reason why metal tea-pots make 
tea so much better than dark earthen ware ones ? 

Mr. P. — It is supposed to be the cause. 

Frederick. — Do hot things, in cooling, part with much 
of their heat in this way ? 

Mr. P. — It depends greatly upon the nature of the body 
cooled ; but, generally, about as much heat is lost by radi- 
ation as by communication. The intensity of radiated 
heat diminishes as the squares of the distance increase ; 
that is, at twice the distance from the radiating body, there 
is only one quarter of the heat. J Therefore a person 
sitting at a distance of two yards from the fire, receives 

* The radii of a circle are lines extending from the centre to the circum- 
ference. The spokes of a carriage wheel are radii ; the nave being the 
centre, and the felloe the circumference, of a circle. — Am. Ed. 

t For this reason, stoves and pipes for heating apartments should be made 
of dark materials ; and vessels, in which liquids are to be kept hot, should 
have their surfaces highly polished.— Am. Ed. 

X The square of a number is the product of that number multiplied by 
itself. Thus 4 is the square of 2, for 2 times 2 is 4 ; and 9 is the square of 
3, for 3 times 3 is 9. So, also, the square of 4 is 16, and the square of 5 
is 25.— Am. Ed. 



20 DIFFUSION OF HEAT. 

only one fourtn part as much heat from it as another person 
sitting at a distance of but one yard. In this respect, and 
indeed in most others, radiant heat is subject to the same 
laws as light, with which it is closely connected.* 

Frederick. — Thank you, father, for telling us so much. 
There is scarcely a day passes that I do not see something 
I cannot understand ; but I think if I could learn as much 
every day as I have learnt this evening about heat, I should 
soon be able to find out many of the things that now 
puzzle me. 

* Radiant heat is reflected in the same manner as light ; but those sur- 
faces which reflect light the most perfectly, are not uniformly the best 
reflectors of heat. A common glass mirror, for instance, when placed be- 
fore a fire, reflects but little heat, and soon becomes hot itself: polished 
metal, on the contra^, reflects the heat very sensibly, and continues quite 
cool. In general, those surfaces which radiate heat the most perfectly, re- 
flect it the least j and the reverse. — Am. Ed. 



QUESTIONS. 



1. What is the use of wooden handles to metallic bodies exposed 
to heat ? 

2. Why does a metallic handle become hotter than a wooden 
one? 

3. Give an experiment to illustrate this fact. 

4. Are paper and woollens good or bad conductors of heat ? 

5. Do different substances, of the same temperature, always feel 
equally hot ? 

6. Why do good conductors of heat become sooner hot and sooner 
cold than bad conductors ? 

7. How can we measure the heat of different bodies ? 

8. What experiment proves that silver conducts heat quicker than 
wood ? 

9. What metal is the best conductor of heat ? 

10. What is said of the conducting power of other metals ? 



DIFFUSION OF HEAT. 21 

11. Is water a good or bad conductor ? 

12. How is heat communicated by hot water ? 

13. What experiment shows that water is a bad conductor ? 

14. Is air a good or bad conductor of heat ? 

15. How does air communicate and take away heat ? 

16. What is meant by the radiation of heat ? 

17. What do } r ou understand by the radii of a circle ? 

18. Upon what does the quantity of heat, radiated by different 
bodies, principally depend ? 

19. What surfaces radiate the most heat ? — What the least ? 

20. By what law does the intensity of radiated heat diminish ? 

21. What do you understand by the square of a number? 

22. In what way is heat reflected ? 

23. What surfaces reflect heat the most perfectly ? 

24. Why does water keep hot longer m a bright than in a dark 
vessel ? 

25. Why will water boil sooner in a dark vessel than in one that 
is polished ? 



22 WARMTH OF CLOTHING. 



CONVERSATION II 



WARMTH OF CLOTHING. 



Mr. Powell's Study. — Mr. Powell, Frederick, 
Robert, and Harriet. 

Mr. P. — After our conversation of yesterday, I dare 
say none of you will have any difficulty in answering why 
woollens and furs are chosen for clothing in cold weather, 
and why metals usually feel so cold. 

Robert. — Oh no ! Woollen cloth is so much warmer 
than metals, that I am sure no one in his senses would 
think of wearing metal near his skin in such weather 
as this. 

Mr. P, — Well, since you are so certain about woollens 
being warmer than metals, take the thermometer, wrap the 
bulb round with wool, and tell me how high it rises. 

Robert. — [After doing as his father directs him.) The 
quicksilver will not rise higher than it was before ; it still 
stands at 50°. 

Mr. P. — Now, then, apply the bulb to the knob of the 
poker, and mark whether the mercury falls. 

Robert. — To be sure it will. (Robert holds the poker 
in one hand, and with the other brings the thermometer close 
to the knob. After holding it in this manner a minute, he 
looks at his father quite astonished.) 



WARMTH OF CLOTHING. 23 

Mr. P. — You seem surprised : — how much has the 
thermometer fallen ? 

Robert. — Not at all ! And instead of falling, as it ought 
to do, — for I am sure the poker feels very cold, — the 
quicksilver is beginning to rise. It is now at 52°, and yet 
the poker seems as cold as ever. 

Mr. P. — Remove the thermometer from the poker, 
again wrap wool round the bulb, and see what follows. 

Robert. — Why, the quicksilver has fallen to 50° ! I'll 
never trust to a thermometer again ; for, though the wool is 
warm and the poker cold, it would have me believe that 
the steel is warmer than the wool. 

Mr. P. — Do not be so hasty in condemning the ther- 
mometer, but rather doubt your own feelings. 

Frederick. — But surely the wool is warmer than the 
cold poker ? 

Mr. P. — The thermometer tells us that it is not ; and 
why should we doubt its accuracy in this case, when we 
know that, in all others, the mercury in the bulb expands, 
and rises up the tube, when we expose it to heat, and that 
it contracts with cold ? I cannot, therefore, suppose it acts 
differently now, but must believe that the poker was hotter 
than the wool. 

Robert. — But it has no feeling, as we have ; and where 
is the use of feeling, if we cannot tell heat from cold better 
than a senseless thing ? 

Mr. P. — It is true the thermometer has no feeling, and 
therefore is less likely to err. Our feelings are acted upon 
by so many circumstances, that it is often difficult to judge, 
from feeling alone, as to the actual heat of any substance. 

Robert. — I don't think I should be so foolish as not to 
tell hot from cold, neither. 

Mr. P. — As you appear so confident in your own 
judgment, we will put it to the test. Fetch three basins, 



24 WARMTH OF CLOTHING. 

and two jugs full of water, one cold from the pump, the 
other hot. (Robert and Frederick bring in the three basins 
and the jugs of water.) 

Harriet. — I am very curious to see this experiment; 
and I should laugh heartily at Robert, if he could not tell 
hot from cold after all. 

Mr. P. — Well, my dear, we shall see. Now, you 
observe, I have poured hot water into the first basin, cold 
water into the third, and a mixture of both into the middle 
basin. The thermometer stands in the first at 120°, in 
the third basin at 40°, and in the middle one at 70°. 
Robert, put your hand into the hot water, and, Frederick, 
put yours into the basin of cold water. 

Robert. — Well, I am sure this is hot; I can hardly 
bear it. 

Frederick. — And I think there can be no doubt that 
this is very cold. 

Mr. P. — Now take your hands out of those basins, 
plunge them quickly into the middle one, and tell me how 
the water in that feels. 

Robert. — Oh ! this is quite cold. 

Frederick. — To me it feels very warm. 

Harriet. — Which of them is right, papa 1 

Mr. P. — Frederick is right in saying the water feels to 
him warm ; therefore Robert must be wrong when he says 
positively it is quite cold. But we shall, perhaps, hear him 
contradict himself. Now, then, my boys, change places : 
and, Robert, put your hand into the cold water, and, 
Frederick, put yours into the hot. 

Robert. — Well, I cannot be mistaken now : this feels 
almost as cold as ice. 

Mr. P. — Take out your hands, and put them into the 
middle basin, as before. Now, Robert, warm or cold ? 

Robert. — Why, it is very warm indeed. 



WARMTH OF CLOTHING. 25 

Harriet. — Warm ! Why, you just now said it was 
cold ! I did not think you " could ever be so foolish as not 
to know hot from cold !" Ha, ha, ha ! 

Robert. — I cannot think it is the same : it must have 
got warmed since I felt it before. 

Frederick. — To me it feels cold. I wish you would 
explain the cause of this to us, father. 

Mr. P. — Yes, I will, presently; but I must first 
convince your brother, that his feelings may not only lead 
him into error, but may be contradictory at the same time • 
for he appears not yet perfectly satisfied. Robert, put 
your right hand into the hot water, and your left into the 
cold, and then plunge them, at the same moment, into 
the middle basin. {Robert, after holding one hand in the 
cold water, and the other in the hot for about a minute, puts 
them together into the middle basin.) Now let us know 
whether it is really hot or cold. 

Robert. — To my right hand it feels cold, and to my 
left hand quite warm. 

Harriet. — Well, Robert, this is worse and worse ! 
Cold one minute, hot the next, and then both hot and 
cold at the same time ! I have heard of a person blowing 
hot at one time, and cold at another ; but you blow both 
at once ! 

Robert. — I am sure it is so ; and you may try yourself, 
Harriet. 

Mr. P. — I have no doubt the water feels, as you say, 
warm to one hand, and cold to the other ; but, as it cannot 
be both at the same time, tell us whether it is warm or 
cold. 

Robert. — Why, it is warm compared with the cold 
water, and cold compared with the hot. 

Mr. P. — Very good ; but still you do not tell us, as 
you were sure you could, whether it is hot or cold. 
3 



26 WARMTH OF CLOTHING. 

Robert. — It feels both. 

Mr. P. — Then you must admit that your feelings, of 
which you boasted, will not enable you to judge, excepting 
by comparison ; and that the same degree of heat may 
appear hot and cold at the same time under different 
circumstances. 

Robert. — It does seem so, indeed. 

Mr. P. — Well, then, since you have had reason to 
doubt the accuracy of your feelings in this experiment, 
I hope you will again trust to the thermometer, and 
believe it was correct in representing the poker to be 
hotter than the wool. 

Robert. — The thermometer was right as to the water ; 
but I cannot think how it could be right as to the poker 
and wool. 

Harriet. — Do tell us how that was. 

Mr. P. — I will now explain the mystery. Every 
inanimate substance, exposed to the same temperature, 
possesses, usually, the same degree of sensible heat ;* and 
the difference in warmth to the touch depends upon their 
different powers of conducting heat, — which was the 
subject of our conversation yesterday. The human 
body, being generally warmer than surrounding objects, is 
continually parting with its heat to them. Now, when 
any thing, that has the power of conducting heat quickly, 
touches our bodies, the heat is drawn rapidly from 
the part touched, and produces the sensation of cold. 
Therefore the poker, though really possessing the same 
degree of heat as the wool, feels very much colder, 
because it has the power of drawing the heat from the 
hand much more rapidly than the wool. 

* By ' sensible heat' is meant that heat which we can feel. Some remarks 
upon the subject of latent heat will be found in a subsequent conversation.— 
Am. Ed. 



WARMTH OF CLOTHING. 27 

Frederick. — But the thermometer did not show this. 

Mr. P. — No, my dear, for the mercury was of the 
same temperature as the wool and the poker ; therefore, 
would not be affected by their different powers of 
conducting heat. But if you make the mercury in the 
bulb hot, you will find that it will descend much more 
rapidly when surrounded by cold metal, than it will if 
wrapped in wool ; that is, the heat will be drawn from 
it more quickly by the metal than by the wool; and, if 
the instrument were capable of sensation, it would feel 
colder with the metal than with the woollen covering. 

Robert. — Ay, but the thermometer not only did not 
fall, but it rose two degrees after being held to the poker. 
How could this happen ? 

Harriet. — Yes, papa ; that made the thing so very odd. 
I should like to know that. 

Mr. P. — The cause of the mercury's rising is easily 
explained. As Robert held the poker in his hand whilst 
I13 applied the bulb of the thermometer to it, the cold he 
felt was produced by the metal's drawing the heat rapidly 
from him, and becoming itself warmer ; which increase 
of heat was shown by the rise of the mercury. 

Harriet. — So that the faster the poker got hot, the 
more positive Robert would have been that it was cold. 
I should never have thought of that. 

Mr. P. — Let us return to the point from whence we 
started : — Can you now tell me why woollens and furs 
are preferred for clothing in cold weather ? 

Frederick. — It must be because they are bad 
conductors of heat, and therefore prevent the warmth 
of the body from being taken away, as it would be by 
substances that were better conductors. 

Mr. P. — Very well explained, Frederick. 

Robert. — Yes, I understand that; but as air is a bad 



28 WARMTH OF CLOTHING. 

conductor of itself, as you told us yesterday, father, why 
should we require any clothing at all to keep ourselves 
warm 1 

Mr. P. — That is, indeed, a very natural inquiry ; and 
I am glad you have asked the question, Robert, as it 
shows you have a spirit of research worthy of a young 
philosopher. I think, however, we have had enough 
of this subject to-day, and it will serve us for our 
conversation to-morrow. 



QUESTIONS. 



1. What experiment is described in the beginning of this conver- 
sation ? 

2. What inference do you draw from this experiment ? 

3. Can we always judge correctly, by the touch, as to heat and 
cold ? 

4. Describe the several experiments which prove this fact. 

5. How are the results of these experiments explained ? 

6. How is the sensation of heat produced ? — How that of cold ? 

7. What is sensible heat ? 

8. Why are woollens and furs preferred for clothing in cold 
weather ? 

9. Why does a person feel cool in summer, after drinking hot tea, 
and warm after drinking iced water ? 

10. What is the comparative temperature of the several articles 
of furniture in the same room ? 

11. Why does the marble hearth feel colder than the carpet? 

12. What is the use of double windows in the winter ? 

13. Give a connected account of the principles taught in this 
conversation. 



COLD PRODUCED BY WIND. 29 



CONVERSATION III 



COLD PRODUCED BY WIND. 



Robert. — Now, father, will you tell us why the air feels 
so much colder than wool, which, if it is a bad conductor 
of heat, it ought not to do? 

Mr. P. — In the first place, before I explain the cause, 
let us perceive the effect ; and for this purpose the bellows 
will do very well. Robert, hold the back of your hand, 
that Frederick may blow upon it with the bellows. {Fred- 
erick blows with the bellows against the bach of Roberts 
hand.) I dare say the wind feels cold. 

Robert. — Yes, very cold indeed. 

Mr. P. — Now, Frederick, let the wind from the bellows 
blow upon the bulb of the thermometer, and see if it pro- 
duces any change. 

Frederick. — [After blowing on the thermometer.) Not 
in the least. 

Robert. — If I had not been convinced, yesterday, that 
my feelings might deceive me, I should certainly say the 
thermometer was not to be depended upon. 

Mr. P. — I am glad you have become less . confident ; 
and, after the experience of yesterday, I trust you will not 
again put your feelings of cold against the decision of the 
thermometer, which now informs us that the wind from the 
bellows and the air of the room are of the same degree of 
heat. 

3* 



30 COLD PRODUCED BY WIND. 

Robert. — Then why do they feel so different? 
Mr. P. — The cold produced by wind is occasioned 
entirely by the motion of the air, which thus presents a 
constantly changing surface to attract the heat from the 
body, — as I mentioned to you in our conversation on the 
diffusion of heat. The air, being a bad conductor, would, 
if at perfect rest, draw the heat from the human body very 
slowly. In that case, the air nearest the surface of our 
bodies would soon become nearly of the same temperature 
as ourselves; and, as it would part with its heat to the 
surrounding portions of air but very gradually, we should 
feel as hot as if covered with the warmest clothing. 

Robert. — Then why is clothing necessary to keep 
ourselves warm when there is no wind ? 

Mr. P. — Because the air is never at rest. The motions 
of our limbs, the movements of our bodies, and our breath- 
ing, all tend to expose us every moment to a fresh surface 
of cold air, independently of other causes. And though 
heat passes with difficulty from one particle of air to 
another, yet each particle, when brought into close contact 
with a heated body, absorbs heat from it ; and the more 
quickly these particles are changed, the faster is the heat 
taken away, and the greater will be the feeling of cold. 

Frederick. — It is in the same way, I suppose, that 
things are cooled by blowing upon them. 

Mr. P. — Exactly so. A rapid change of air always 
produces a feeling of cold when the temperature of the 
atmosphere is lower than that of our bodies. Even the 
thermometer may be made to exhibit signs of being affected 
by a current of air, as I can show you by an easy ex- 
periment. 

Harriet. — I should like to see that senseless thing 
made to feel cold by the wind. 

Mr. P. — Hold the bulb of the thermometer, Robert, in 



COLD PRODUCED BY WIND. 31 

hot water till the mercury rises to 120°, and then take it 
out, and notice how long it is in falling to 80°. 

Robert. — (Looking at his watch with the thermometer in 
his hand.) Just three minutes. 

Mr. P. — Raise the mercury to 120° again ; and be you 
ready, Frederick, with the bellows, to blow upon the bulb 
as soon as Robert takes it out of the water. 

Robert. — (Holding the thermometer, while his brother 
blows tts fast as he can with the bellows.) The quicksilver 
has fallen to 80° in one minute only. 

Mr. P. — You would find, on repeating the same experi- 
ment with the bulb well wrapped in wool, that it would 
then resist the action of the bellows much longer ; for the 
wool, besides being a bad conductor itself, would prevent 
the air nearer the bulb from being so quickly changed 
by the bellows. 

Frederick. — Do our clothes keep us warm in this way, 
by protecting us from the wind ? 

Mr. P. — Yes, my dear ; and this is the principal use of 
the thick clothing we put on in winter ; for if we could 
enclose ourselves in a case perfectly air-tight, a thin flannel 
covering might keep us as warm as the thickest great 
coats. The most effectual mode of keeping out the wind, 
and preserving the heat of the body, is that adopted by the 
inhabitants of the polar regions, who wear the skins of 
animals turned inside out. By this means the dried skins 
act as a screen to prevent the wind from penetrating ; and 
as the fur contains a quantity of air between its fibres, that 
part of the body so covered is thus enclosed in a case of 
confined air. 

Frederick. — But if the air were hotter than the body, 
would the wind feel cold ? 

Mr. P. — No, Frederick. When the temperature of the 
air is greater than that of the body, the wind feels insuiFer- 



32 COLD PRODUCED BY WIND. 

ably hot. The hot winds from the deserts of Arabia and 
Africa often destroy the lives of animals exposed to them ; 
and the natives, who know when to expect these winds, 
shut themselves up in their tents to be out of their scorch- 
ing effects. 

Harriet. — I should like to feel a hot wind. 

Robert. — I can't think how it is possible for the air 
to be hot. 

Mr. P. — I believe I can satisfy your doubts and Har- 
riet's curiosity very shortly. Frederick, light the Argand 
lamp, — put on the glass chimney, — and then give me the 
bellows. Now, you observe that I have closed the nozle 
of the bellows, and that I hold the large opening, which 
admits the air, at a little distance from the chimney of the 
lamp, so that, when I separate the legs slowly, the hot air, 
rising from the lamp, is drawn in. I must expel the air, 
and fill the bellows a few times, to heat the interior, and 
drive out the cold air. Now, Harriet and Robert, hold 
your hands, and feel the wind as it is forced out of the nozle. 

Robert. — It is quite hot ! 

Harriet. — Yes, so it is! I declare the bellows can 
blow hot and cold as well as you, Robert. 

Frederick. — I suppose the cold produced by fanning 
is owing to the motion given to the air by the fan. 

Mr. P. — Yes, it is. By the motion of the fan, fresh 
particles of air are driven rapidly against the face, and 
thereby carry away a greater portion of its heat than 
before. You may frequently hear ladies, who are fanning 
themselves, say, that they are " cooling the air," though, 
in reality, every motion of the fan gives additional heat 
to the air, by causing it to abstract a greater quantity 
from the body. 

Harriet. — That is very curious. I always thought 
it made the air cooler. 



COLD PRODUCED BY WIND. 33 

Mr. P. — Fanning makes the face cooler, my dear, 
but it does so only by carrying off its heat more rapidly 
to the air. 

Robert. — But fans are generally used in hot rooms 
and in warm weather, when the air is already so hot that, 
if it were not cooled by the fan, it would be like a hot 
wind. 

Mr. P. — The air is not, in the hottest room, nor in the 
warmest weather, in this country, ever so hot as the 
human body. The natural heat of the body is 98°, 
while our hottest summer's day seldom exceeds 84° in 
the shade. The air, however, is never heated so high 
as the temperature indicated by the thermometer ; for 
that instrument is acted upon by the heat reflected from 
surrounding objects ; but even if we allow the air to 
contain 84° of sensible heat, it would still be considerably 
cooler than the body, and a strong current, by greatly 
increasing its conducting power, would feel very cold. 

Harriet. — Then it seems that ladies, who fancy they 
are cooling the air by fanning, make the same mistake 
that Robert did, when he thought the poker was so very 
cold because it was getting hotter. 

Mr. P. — Exactly so. I trust you now understand the 
cause of the air feeling generally cold ; and why the 
sensation of cold is increased on its being put into rapid 
motion. When I speak of cold, however, you must not 
imagine that I mean a total absence of heat ; and still 
less, that cold is an absolute property, capable of entering 
into different substances, like heat. After what you have 
heard, you may, perhaps, be prepared to learn, that there 
is no such thing as absolute cold. 

Robert. — Not such a thing as cold ! "Well, father, you 
might be right about the poker and wool ; but as to cold, 
nobody can be mistaken that can feel. 



34 COLD PRODUCED BY WIND. 

Mr. P. — What, Robert ! do you still place such 
reliance on your feelings ; and have you so soon forgotten 
how they failed you, when put to the test with the hot 
and cold water? 

Robert. — No ; I have not forgotten that ; but to 
suppose there is no such thing as cold, one must forget 
to feel. 

Harriet. — I cannot help thinking as Robert does 
about cold. Do, pray, papa, tell us what it is you mean. 

Mr. P. — I will do so, my dear, to-morrow. 



QUESTIONS. 

1. With what experiment does this conversation begin ? 

2. What do you infer from this experiment ? 

3. Why is it colder when the wind blows than in a calm ? 

4. Is the air ever perfectly at rest ? 

5. Why does blowing upon a hot thing cool it ? 

6. Is the thermometer ever affected by a current of air ? In what 
manner ? 

7. How do clothes keep us warm ? 
8.. What is said of hot winds ? 

P. In what manner does fanning cool one ? 

10. Is the air cooled by being put in motion ? 

11. Why doe3 a person feel cooler when riding fast, on a hot day, 
than when sitting still ? 

12. What is the ordinary temperature of the human body ? 

13. Is the air ever hotter than the human body in this climate ? 

14. If the air were hotter than the body, what would be the 
effect of fannino- ? 



COLD. 35 



COX VERS AT I OX IV. 



COLD. 



(Frederick, Robert, and Harriet, warming their 
Hands at the Fire. The Morning very cold and frosty, 
and a heavy Fall of Snow. Mr. Powell enters the 
Room.) 

Harriet. — Papa, I am glad you are come at last. 
Now, when the ground is covered with snow, and we 
are almost frozen into icicles, do you still say, there is 
no such thing as cold \ 

Mr. P. — It feels intensely cold, my love ; but our 
feelings, as Robert knows, may be mistaken : and I still 
say, and hope to prove to you, that there is no such 
as cold. 

Frederick. — What is it, then, that makes the water 
freeze, and that makes us all feel so cold to-day? 

Mr. P. — The short and correct answer to that question 
is, that the air is not so hot to-day as it was yesterday. 

Robert. — But it was not hot yesterday, by any means : 
it was only not so cold as it is now. 

Mr. P. — The air did not appear to us to contain any 
heat, because our bodies were so much hatter than the 
air, and, consequently, it deprived us of heat: but it 
certainlv contained more sensible heat than the air 



36 COLD. 

does today; and I think I can make you aware that 
this frosty day is not without heat. 

Harriet. — Do show us how ; for I am now shivering 
with cold. 

Robert. — With want of heat, you should say, Harriet. 

Mr. P. — Yes, Robert, ridiculous as it may sound, that 
is correct. Even snow feels warm when compared 
with substances still colder, as I can convince you if 
you bring me a basin full of snow and a cup full of 
salt. (Robert brings in the snow and salt, as required.) 
Now put the thermometer into the snow, and see the 
temperature. 

Robert. — It is 32°. 

Mr. P. — That is just the freezing point ; and it is, 
therefore, as cold as ice. I will now put part of the 
snow into another basin, and mix the salt with it in 
the proportion of one part of salt to two of snow : 
observe, the mixture is become liquid. Robert, put 
your hand into it. 

Robert. — [Putting Ms hand into the water.) Oh! oh! 
I cannot bear it any longer. 

Mr. P.^Take your hand out, then, and put it into 
the snow, and tell us how that feels. 

Robert. — Why, compared with the other, it is warm. 

Mr. P. — Harriet, put your hand into the snow that 
Robert says feels warm, and let us know what you 
think of it. 

Harriet. — Oh ! it is very, very cold. 

Mr. P. — Nay, Harriet, it cannot be really cold and 
warm at the same time. Robert says the snow is warm 
compared with the mixture of snow and salt, and the 
thermometer will agree with him. (Mr. Powell puts 
the thermometer bulb into the basin of snow and salt.) 
See, the mercury falls 32° below the freezing point: 



COLD. 37 

fcnd when I remove it to the snow, it will rise to 32° ; 
therefore Robert must be right in saying the snow is 
warm. It felt cold to you, Harriet, because the air of 
the room is so much warmer than snow; but Robert, 
having previously immersed his hand in a mixture so 
much colder, was capable of feeling the heat of the snow. 

Robert. — Yes, father ; but the mixture of snow and 
salt must be cold ; for the thermometer sunk down to 
nothing, and there cold must begin. 

Mr. P. — It was, indeed, so considered by Fahrenheit,* 
the maker of the thermometer generally used in this 
country ; but recent observations have proved that opinion 
to be wrong. The thermometer may be made to fall 
50° below the nothing in Fahrenheit's scale by a mixture 
of snow and potash ; and, compared with this, the mixture 
of snow and salt would be very perceptibly w T arm. 
It would be dangerous, however, to expose the flesh 
suddenly to contact with a mixture of so low a degree of 
temperature : but the thermometer would show that the 
snow and salt contained heat, by its rising rapidly in 
that mixture when taken from the snow and potash. 

Frederick. — How cold is it in the coldest part of 
the world ? 

Mr. P. — The temperature near the poles we cannot 
ascertain, as the solid masses of ice, that constantly cover 
that part of the globe, prevent ships from approaching 

* There are three kinds of thermometers in common use, viz. Fahrenheit's, 
which is used in England and North America. In this thermometer, the 
freezing point is at 32°, and the boiling point at 212°. 

Reaumur's, which is used in Switzerland, Italy, and a part of Germany. 
The scale of this thermometer begins at the freezing point, and the boiling 
point is at 80°. 

The Centigrade, which is used in France, Sweden, and Denmark. On 
the scale of this thermometer the freezing point is marked 0°, and the 
boiling point, 100°.— Am. Ed. 

4 



38 COLD. 

within several hundred miles. The voyagers, who 
have approached the nearest to the poles, have found 
the cold so intense as to become dangerous, notwith- 
standing all the precautions they could take. Even at 
Hudson's Bay, the thermometer is frequently as low as 
53° less than nothing; compared with which our present 
temperature, that you think so cold, would be like the 
hottest day in summer. 

Harriet. — How can the poor people manage to keep 
alive there ? 

Mr. P. — They wrap themselves in furs, and endeavor, 
as much as possible, to avoid exposing any part of their 
bodies to the air. The snow, too, with which the ground 
is covered, helps to keep their cabins warm. 

Harriet. — Snow make them warm! Well, that is 
very odd ! 

Mr. P. — Yes, my dear ; the air confined between the 
flakes or crystals of snow being a very slow conductor of 
heat from the body, the inhabitants find that, when rolled 
up in a blanket, and buried under the snow, they are 
warmer than in their beds. The temperature is so low in 
that part of the world, that, if water be thrown into the air, 
it will be ice when it falls down ; and it is even stated, 
that the moisture of the breath is sometimes frozen, and 
falls to the earth like a shower of snow. 

Harriet. — What, breathe a shower of snow ! 

Mr. P. — After having shown you that what feels to us 
intensely cold, really contains heat, and is absolutely warm, 
compared with colder temperatures, I trust your confidence 
in the existence of cold is shaken. 

Frederick. — But if there be no such thing as cold, I 
suppose there must be a point at which there is no heat ; 
and how low must the thermometer fall when all the heat 
is taken away? 



COLD. 39 

Mr. P. — There is, no doubt, a point at which all bodies 
would be deprived of heat; but that point has never been 
ascertained. The lowest temperature hitherto produced 
is 100° less than zero, or 132° below the freezing point. 
Brandy freezes at 7° below zero ; mercury becomes solid, 
like other metals, at 39°, and ether is congealed at 46° 
below zero : it is supposed that even the air we breathe 
would become a solid mass if the temperature could be 
sufficiently reduced. 

Robert. — But if it were possible to take away all the 
heat from a thing, it must be cold then. 

Mr. P. — No, Robert, your conclusion is by no means 
correct. It does not follow, as a matter of course, when 
one property is taken from a body, that another, and 
opposite one, must exist and enter into it. I think I 
have convinced you, that, even when the temperature is so 
low as to deprive human beings of life, the air contains 
heat ; therefore, if that degree of temperature be not cold, 
we have no reason to suppose such a property as cold 
exists. Persons, naturally enough, imagine all negative 
properties to have a positive existence ; which opinion, 
however, philosophy has proved to be erroneous : but from 
custom, and for general convenience in conversation, the 
words which express the negative quality are used, even 
by those who do not attach any positive meaning to them. 
Thus, cold, dulness, darkness, and many other expressions 
of the kind, are commonly used, though only intended to 
signify a deficiency of heat, of brightness, or of light, &,c. 

Frederick. — But would it not be better to call things 
by their right names? for if we know there is no such 
thing as cold, why should we talk as if there were ? 

Mr. P.— I agree with you, Frederick, that the use of 
words which convey a wrong idea of the intended meaning 
is a bad practice, but it is a difficult one to alter. Even 



40 COLD. 

men of undoubted science would shrink from the imputa- 
tion of learned affectation, which such a departure from 
generally received expressions would bring upon them ; 
and while they refuse to sanction such an alteration in 
established modes of expression, it would be great pre- 
sumption in others to attempt to introduce a change in this 
respect. Any kind of affectation is, at the best, ridiculous ; 
but an affectation of learning, especially in young persons, 
is sure to excite the dislike as well as the ridicule of every 
one. I trust, my dear children, I shall never observe this 
affectation in you ; for I had rather you should remain 
ignorant, than that you should pretend to be learned. 



QUESTIONS. 

1. Is there any such thing as cold, regarded as a positive 
property ? 

2. Does snow contain any heat ? 

3. What experiment proves this ? 

4. At what point of the thermometer does water freeze ? 

5. How low does the mercury fall in a mixture of salt and snow ? 

6. Does not cold begin at 0°, or zero ? 

7. How many kinds of thermometers are used ? 

8. In what particulars do they differ from each other ? 

9. How low will the mercury fall in a mixture of snow and 
potash ? 

10. What is said respecting the cold of the polar regions? 

11. At what degree does mercury become a solid metal ? 

12. Is it possible to remove all heat from a body ? 

13. What is the lowest temperature that has been produced ? 

14. If all heat could be removed from a body, would it then 
contain the property of cold ? 

15. What is cold ? — dulness ? — darkness ? 

16. Why should the same temperature seem colder at one time 
than at another ? 

17. Repeat the leading principles taught in this conversation. 



EXPANSION BY HEAT. 41 



CONVERSATION V 



EXPANSION BY HEAT. 



Frederick. — What a cold morning this is, father ! The 
thermometer is 6° below the freezing point. 

Harriet. — I have been puzzled, papa, to imagine how 
the thermometer can tell when it is hot and when cold, 
as it cannot feel. 

Mr. P. — It is a question that, I dare say, has puzzled 
many older heads than yours, Harriet ; and as we have 
made so much use of the thermometer lately, it will be 
advisable to make you understand the principle which 
regulates the rise and fall of the mercury in the tube. 
It depends upon the expansion which the quicksilver 
undergoes by heat, and its contraction when the heat is 
removed. 

Harriet. — Does heat make the same quantity of 
quicksilver really larger than it was before 1 

Mr. P. — Yes, that is its effect, as the thermometer 
itself might inform you. But, lest you should imagine 
there is any hidden virtue in the frame and bulb, you 
shall see the same effect produced on mercury in a 
straight tube. I will put into this tube an ounce of 
mercury, and make a mark on the glass at the point it 
reaches in the tube. I will now immerse it in hot water, 
and observe the effect of the heat. 
4* 



42 EXPANSION BY HEAT. 

Frederick.— The quicksilver has risen above the 
mark already ; I see it rising very distinctly. 

Mr. P. — As it now occupies more space in the tube, 
you must be convinced that it has increased in bulk. 

Frederick. — Does it weigh more now than it did 
before 1 

Robert. — Of course, a larger quantity of quicksilver 
must weigh heavier than a smaller. 

Mr. P. — Weigh it yourself, then, Robert, and tell us 
how much weight it has gained. 

Robert. — (Weighing the mercury.) It weighs exactly 
an ounce. 

Harriet. — Then it is no heavier than when first put 
into the tube. Is that as it should be ? 

Mr. P. — Yes ; Robert's position, that equal bulks of 
the same thing must always weigh the same, will not 
hold good, as he must now perceive. 

Robert. — But something must have got into the 
quicksilver, to make it larger than it was before, and that 
something must, I think, make it heavier. 

Mr. P. — The " something " you speak of is heat, 
which is, indeed, supposed to be a material substance ; 
but philosophers have not yet been able to discover that 
it has any weight. Heat possesses the property of 
expanding all substances ; and it is supposed to do this 
by surrounding the minute particles of which the 
substances are composed, and thus separating them 
farther from each other. 

Frederick. — Then I suppose the quicksilver must 
be really lighter when made hot than it is when cold. 

Mr. P. — Yes ; the weight of the same quantity of any 
substance is diminished in proportion to the expansion. 
Thus, if ten inches of mercury be expanded to eleven 
inches, the snecifir. orravitv win v> e nearly one tenth less 



EXPANSION BY HEAT. 43 

than before ; that is, eleven inches of the expanded 
mercury will weigh no more than the ten inches before 
expansion. 

Harriet. — But how can the bulb of the thermometer 
be filled with quicksilver through so small a tube as it 
appears to be ? 

Mr. P. — I will show you in a moment. The difficulty, 
though seemingly great, is soon overcome. (Mr. Powell 
fetches from his cabinet an empty thermometer tube.) 
The aperture of this tube is so small, that it might be 
considered impossible to fill the bulb at the end. 

Harriet. — I can scarcely see the opening. I am 
quite curious to know how you will manage to pour the 
quicksilver through it. 

Mr. P. — I shall make the mercury run into the tube 
without the least trouble. (Mr. Powell lights a candle, 
and holds the bulb near to the flame.) I have now heated 
the bulb sufficiently, and will plunge the open end of the 
tube into this cup of mercury. Look at the tube as the 
bulb cools. 

Harriet. — The quicksilver is running up the tube 
by itself, and is entering the bulb. How very strange ! 

Mr. P. — Having now got the bulb nearly half full, I 
will hold it again to the candle till the mercury boils, 
and, on again plunging the tube into the cup, the bulb 
will be filled. (Mr. Powell fills the bulb in the manner 
described, at which the children seem greatly astonished.) 

Frederick. — Do tell us, father, what is the cause of 
the quicksilver running up the tube. 

Mr. P. — Yes, my dear ; and the more readily, as it 
affords us another example of the expansion caused by 
heat. When I held the empty bulb to the candle, the heat 
expanded the air, and drove part of it through the tube. 
After I had placed the tube in the cup of mercury, the 



44 EXPANSION BY HEAT. 

air in the bulb, on becoming cool, contracted ; and, as 
the mercury prevented the external air from rushing in 
to occupy its former space, the liquid metal was forced 
up to supply its place. A more perfect vacuum was 
afterwards produced by the condensation of the vapor 
of the boiling mercury, and the whole bulb was filled. 

Robert. — Then air expands by heat, as well as 
quicksilver. 

Mr. P. — Yes, Robert ; not only quicksilver and air, 
but every substance, visible or invisible, is expanded by 
heat. The expansion of liquids and air is, indeed, 
greater than that of solid bodies ; but the hardest rocks 
and metals are also subject to the expansive power of heat. 

Robert. — How can we tell that the air is expanded 
by heat 1 

Mr. P. — The experiment already shown you, with the 
thermometer bulb, is a conclusive proof that it is so ; but 
the expansion may be rendered visible by the inflation of 
a fire balloon. 

Harriet. — Do show us that, papa ; balloons are such 
curious things. 

Mr. P. — I am glad I can gratify your curiosity, and 
exemplify an interesting law of nature at the same time, 
Harriet. {Mr. Powell brings from his laboratory a small 
fire balloon.) This balloon is made of silver paper per- 
fectly air-tight, excepting at the bottom, where there is an 
opening and a small iron-wire car to hold the combusti- 
bles by which it is to be inflated. I shall put into the car 
cotton steeped in spirits of wine. The balloon is at pres- 
ent, you perceive, much collapsed, and looks shrivelled. 
Robert, apply a piece of lighted paper to the cotton, as I 
hold the balloon, and notice the effect. 

(Robert sets fire to the spirits of wine, and in a short 
time the sides of the balloon begin to expand.) 



EXPANSION BY HEAT. 45 

Frederick. — The balloon begins to move, as if it were 
being blown out like a bladder. 

Harriet. — Its sides are now on the full stretch, and it 
seems quite swelled out. 

Mr. P. — The " swelling out " is occasioned by the 
expansion of the air inside, which, owing to its being 
kept in an expanded state by the heat of the burning 
spirits, is become lighter than the air of the room ; and 
when I remove my hand, the balloon will ascend. (3Ir. 
Powell permits the balloon to rise to the top of the 
room.) 

Harriet. — How very pretty it looks, rising up without 
any one touching it ! 

Frederick. — I suppose, then, father, that air, when 
heated, becomes lighter than before, in the same manner 
as the hot quicksilver did, because the quantity of it is in- 
creased without any additional weight. 

Mr. P. — Exactly so. But look! the spirits are nearly 
burnt out, and the sides of the balloon begin to collapse, 
as the heat is not sufficient to expand the air. It is now 
falling to the ground. This experiment shows, in the 
most striking manner, the effect of heat in the expansion 
of air. The ascent of smoke up chimneys is owing to this 
effect of heat. 

Frederick. — How is that, father ? 

Mr. P. — The heat of the fire expands and rarefies the 
air in the chimney, and as it is thus made lighter than 
the external atmosphere, it rises. Its place is supplied 
by fresh air from below, which is heated in the same 
manner. The whole chimney thus becomes filled with 
a column of air much lighter than a column of the atmos- 
phere of the same height, and it therefore issues rapid- 
ly from the top, while the air below rushes to the fire- 
place. • 



46 EXPANSION BY HEAT. 

Frederick. — Then the draught* of a tall chimney 
should be greater than that of a short one, as the column 
of light air must be higher. 

Mr. P. — That is, generally, the case, provided the fire 
is sufficiently large to keep the whole column heated. 

Robert. — But the chimneys of very high houses will 
sometimes smoke, which they ought not to do, if the 
draught depends upon the height of the column of light air. 

Mr. P. — Many causes may tend to make even a very 
tall chimney smoke. Chimneys are often so badly con- 
structed as either to prevent the air from becoming suffi- 
ciently heated, or to obstruct its free passage upwards. 
For instance, if the opening above the grate be very large, 
it will admit more air than the fire can heat, and the 
column in the chimney will not be sufficiently light to 
ascend as it ought to do; or, if there be many irregular- 
ities in the chimney itself, or any accidental obstruction, 
the progress of the air and smoke will be partially stop- 
ped, and part of the smoke will enter the room. 

Harriet. — If the fire is the cause of the draught, what 
is the use of putting those things on the tops of chimneys 
to prevent their smoking ? 

Mr. P. — They are only intended to act as screens 

* The common remark, that chimneys draw smoke, conveys a wrong 
impression ; as it implies that the smoke is carried up by suction. Smoke 
consists of vapor, gases, and minute particles of the fuel, which are taken up 
by the current of hot air in the chimney, in the same manner as a running 
stream of water will bear along any light substances thrown into it. The 
chimney, therefore, does not draw the smoke ; but only suffers the hot air 
to ascend, carrying the smoke along with it. A tight room must, of course, 
be smoky ; for the current up the chimney must fail for the want of a suffi- 
cient supply of air. Hence the necessity of opening windows and doors. 
Fire-places may be so constructed as to keep the room fully supplied with 
air, heated before it is admitted. An arrangement for this purpose con- 
tributes so much to economy, comfort, and health, that it should never be 
dispensed with. — Am. Ed. 



EXPANSION BY HEAT. 47 

against the wind, and not to increase the draught ; for if 
the wind blow into a chimney, it has the effect of ob- 
structing the current of air from issuing out at the top, 
and of driving it down. 

Frederick. — I understand now why it is that chim- 
neys generally smoke in a high wind. 

Harriet. — I have seen a piece of paper cut and paint- 
ed like a snake, hanging with its head downwards, and 
supported with a wire by its tail over a candle, that turned 
round and round as if it were alive : was its turning round 
owing to the expansion of the air by the candle ? 

Mr. P. — Yes, my dear. The heat of the candle rare- 
fies the air, and causes it to rise, producing an upward 
current. This current of air, striking against the oblique 
sides of the paper snake, gives it a rotary motion, and the 
paper will continue turning as long as the candle pro- 
duces the current. 

Frederick. — Do smoke-jacks in chimneys turn round 
from the same cause 1 

Mr. P. — Yes; the current of air up the chimney 
strikes against the oblique vanes of a wheel fixed there, 
with sufficient force to turn it round, and put in motion 
all the machinery of the smoke-jack. Many natural phe- 
nomena are produced by the action of heat in expanding, 
and consequently lessening the weight of, the air. The 
winds are caused, by this effect, as I shall explain to you 
more particularly on some future occasion. I will con- 
clude this morning's conversation by mentioning to you a 
few instances of the expansion of other substances by the 
same means. This knitting needle will answer our pur- 
pose exceedingly well to show the expansion of metals. 
You see that it is just large enough to enter the ward of 
this key : I will put it into the fire to get hot, and you 
shall try if it will pass through then. Here, Robert, take 



48 EXPANSION BY HEAT. 

hold of the needle with this piece of paper, and try to put 
it into the ward. 

Robert. — ( Trying to get the knitting needle into the 
ward.) It will not enter the ward now ; it has grown 
larger than it was. 

Mr. P. — Yes ; it is expanded by the heat. 

Robert. — You told us, father, that the expansion of 
quicksilver by heat was owing to the particles of the quick- 
silver being separated by the heat getting between and 
surrounding them : how is it that heat expands other metals 
whose particles do not move at all ? 

Mr. P. — The particles of solid metals, indeed, appear 
to be immovable ; but they are supposed to be continually 
in motion,* though they are so minutely small that their 
motions are imperceptible. Heat, it is concluded, operates 
in the same way upon solid substances as upon liquids, by 
separating their particles or atoms ; and that some motion 
must have taken place in the metal of the knitting needle 
is evident from the fact of its being expanded in all di- 
rections. 

Frederick. — Are all metals expanded as much by heat 
as iron is? 

Mr. P. — No : the metals, and nearly all substances, 
vary in the degrees of their expansion ;f but it is a general 

* That is, when they are undergoing- a change of temperature. — Am. Ed. 
t The following table shows how much some common substances increase 

in bulk, when their temperature is raised from that of freezing to that of 

boiling water. 

Glass gains one part in 416 

Steel 283 

Iron 271 

Gold 221 

Copper 194 

Brass 177 

Silver .". 175 

Tin 170 



EXPANSION BY HEAT. 49 

rule, that bodies expand by heat, and contract with cold. 
The most remarkable exception to this rule is water. 

Frederick. — Is not water expanded by heat, father ? 

Mr. P. — Yes ; it is expanded by all temperatures above 
4'2° ; but on being cooled below that point, it begins to 
expand again, and the expansion increases as the temper- 
ature is reduced below 42°, in the same proportion as it 
increases when it is heated above that point. 

Harriet. — That is very singular : what can be the 
reason of it ? 

Mr. P. — It is not known in what manner the abstrac- 
tion of heat from water causes its expansion below 42° ; 
but, ignorant as we are of its cause, we know sufficient to 
excite our admiration of the beneficent provision of Nature 
in making this exception to the general laws which regu- 
late other bodies. If water were contracted as it is 
deprived of heat, like other substances, the ice, as it was 
formed, would sink to the bottom of the water, a fresh 
quantity would supply its place, and, being frozen, sink as 
before ; and, in a continued frost, the whole of our rivers 
and northern oceans would become solid masses of ice. 
In consequence of this singular exception, however, as 
soon as the water is cooled below 42°, it swims on the 
surface, and protects the water below from being exposed 
to the freezing influence of the atmosphere. 

Robert. — But the water immediately underneath must 

Lead gains one part in 117 

Mercury 55 

Water 23 

Alcohol 9 

Common Air. 3 

From this table it will be seen, that a piece of glass, which measures 416 
cubic inches at 32° of the thermometer, measures 417 cubic inches at 212° 5 
while 3 cubic inches of common air becomes 4 cubic inches, by the same 
change of temperature.— -Am. Ed. 

5 



50 EXPANSION BY HEAT. 

touch the ice on the top, and therefore become as cold 
as it is. 

Mr. P. — It does so to a certain extent, and that occa- 
sions the thickness of the ice in long-continued frosts ; but 
water is so slow a conductor of heat, that it requires a 
length of time to abstract the heat from the water below, 
so long as it remains stationary there by its superior 
weight. 

Robert. — Then, father, we should have had no 
skating, if water got heavier as it became colder, until the 
whole water in the ponds were frozen. 

Mr. P. — No, indeed, Robert, you would not ; for the 
ice would be at the bottom. 

Frederick. — Nor any fishing either, for all the fish 
would be killed by the first hard frost. 



QUESTIONS. 



1. What makes the mercury rise and fall in the thermometer ? 

2. Does heat make a given quantity of quicksilver larger ? 

3. What experiment proves this ? 

4. What effect has the expansion of the quicksilver upon its 
weight ? 

5. Has heat any weight ? 

6. How is it supposed to expand bodies ? 

7. In what way is the tube of a thermometer filled with quick- 
silver ? 

8. What is the cause of the quicksilver running up the tube ? 

9. What effect kas heat upon air ? 

10. What substances are the most expanded by heat ? 

11. What experiment shows that the air is expanded by heat? 

12. Is the weight of the air affected by its temperature ? 

13. To what is the ascent of smoke in chimneys owing ? - 

14. Name some of the causes that tend to make chimneys 
smoke. 



EXPANSION BY HEAT. 51 

15. What is the use of the pots and other contrivances which we 
see on the tops of chimneys ? 

16. Why should a tali chimney carry smoke better than a short 



one 



17. When a chimney does not carry smoke well, why should we 
open a door or window ? 

18. Describe the spiral serpent — what makes it move ? 

19. Explain the motion of a smoke-jack. 

20. Is iron expanded by heat ? 

21. How is this proved ? 

22. In what way does heat expand solid bodies ? 

23. Are the different metals expanded equally ? 

24. Does water expand or contract in freezing ? 

25. Is the cause of this phenomenon known ? 

26. Give an account of the changes which water undergoes in 
freezing. 

27. What remarks are made on this provision of Nature ? 

28. Why are pitchers, decanters, &c. broken when water freezes 
in them ? 

29. Give an analysis of the leading principles explained in this 
conversation. 



52 BOILING. 



CONVERSATION VI 



BOILING. 



Harriet. — Oh, look, papa ! the kettle is boiling over. 
I was afraid I should have been scalded. 

Mr. P. — Have you ever thought what it is that causes 
the water in the kettle to boil over in this way ; or of the 
cause of its boiling at all ? 

Robert. — It does not require much thought, I suppose, 
to tell that : it is the fire. 

Mr. P. — We should not be much wiser by that answer, 
Robert. The fire makes the water boil — but how ? 

Robert. — By making it hot. 

Mr. P. — We are no nearer to an explanation yet. 
The question is, How does heat produce the effect 1 

Robert. — That I cannot tell ; but I know that water 
will not boil without fire. 

Mr. P. — Do not be too positive, Robert. What would 
you say to a proposal for boiling water by cold ? 

Robert. — That it is impossible. 

Mr. P. — I will convince you to the contrary. 

Harriet. — Can you, papa ? I am all curiosity to see 
how. 

Mr. P. — Bring me a thin bottle, or decanter, and I 
will half fill it with water from the kettle. (Robert 
fetches his father a decanter, into which Mr. Powell 



BOILING. 53 

pours boiling icater, after first warming the decanter , to 
prevent it from cracking, and then he corks it.) You 
perceive the water is perfectly motionless, and does not 
boil now. Robert, bring me a cloth dipped in the coldest 
water you can find. It is to make the water boil. 

Robert. You must be joking, father. 

Mr. P. — Bring the cloth, and you shall see. (Robert 
goes for the loct cloth, and brings it to Mr. Powell.) 
I will put it to the top of the bottle, which is full of 
steam ; now look, Robert, at the water. 

Robert. — Why, it is really boiling as briskly as if it 
were on the fire. 

Mr. P. — You see that when I remove the cold cloth, 
the water ceases to boil ; and that when I cover the neck 
with the cloth, it boils again. 

Frederick. — What is the cause of its doing so, father ? 

Mr. P. — Before I explain the cause of this phenomenon, 
I must return to the original question ; — What makes 
water boil when put on the fire 1 

Robert. — I do not know what to say ; I will thank 
you to tell me, father. 

Mr. P. — When water is placed in the kettle, over the 
fire, a portion of heat is communicated to it every 
moment, and the water at length becomes heated to the 
boiling point, that is, to 212° of Fahrenheit's thermometer. 
At this point, the water nearest the bottom of the kettle 
is converted into steam, which, as soon as it is formed, 
rises rapidly to the surface in large bubbles, causing great 
commotion in the water ; and, when the fire is hot, the 
steam is formed so quickly, that the bubbles force part 
of the water above the top of the kettle, causing it to 
boil over. 

Harriet. — But the steam begins to form some time 
before the water boils. 
5* 



54 BOILING. 

Mr. P. — Yes, my dear, but very slowly ; for the elastic 
force of steam does not become sufficiently strong to 
balance the pressure of the atmosphere, until the water 
is heated to about 212° ; and the steam will not form at 
the bottom until its force is equal to the atmospheric 
pressure. 

Frederick. — How does the pressure of the atmosphere 
prevent the steam from rising ? 

Mr. P. — In the same way as a weight placed upon an 
elastic body presses it down, and prevents it from rising. 
Water, when possessing sufficient heat to keep it in a 
liquid state, has a strong tendency to evaporate; and, 
were it not for the pressure on its surface, it would be 
rapidly dissipated into an elastic vapor. The force of 
the vapor of water, at 32° is equal to one ounce and a 
half on every square inch ; and as the water is heated, the 
force of the vapor increases, until, at 212°, it is equal 
to the pressure of the atmosphere, that is, fifteen pounds 
on every square inch; and the steam then rises to the 
surface as fast as it is formed. After water is heated to 
its boiling point, it cannot be made any hotter so long as 
it is exposed to the air, however hot the fire may be ; 
for the heat, as fast as it enters the water, is conveyed 
away by the steam. If water be heated under a greater 
pressure than that of the atmosphere, the temperature may 
be raised considerably higher than 212°, without boiling; 
and, on the contrary, if the pressure of the atmosphere be 
removed, water will boil at 72°. It has been ascertained 
that fluids generally boil in a vacuum at a temperature 
140° less than under the pressure of the atmosphere. 

Harriet. — But, now, tell us how it was that the water 
in the bottle boiled by cold ? 

Mr. P. — When I applied the cold cloth to the upper 
part of the bottle, the steam at the top became condensed 



BOILING. 55 

into water, owing to the absorption of its heat by the cloth, 
and produced a partial vacuum in the bottle. Part of the 
pressure of the atmosphere being thus removed, the elastic 
force of the steam, formed by the hot water, became more 
than equal to the remaining atmospheric pressure, and there- 
fore it rose in bubbles to the surface. When I removed the 
cold cloth, the steam ceased to be condensed, and, therefore, 
shortly pressed upon the hot water with a force equal to 
the elastic power of the steam it was capable of forming, 
and prevented the steam from rising ; but on re-applying 
the cold cloth, the same effect was repeated, from the 
same cause. 

Robert. — Ay, but the water was nearly boiling hot to 
begin with : you cannot make liquids boil without heating 
them first. 

Mr. P. — It is true that water will not boil, even under 
the receiver of the air-pump, till it is heated to 72° ; but 
ether, which is a more volatile fluid, can be made to boil 
without being heated.* 

Frederick. — I should like to see it very much. 

Mr. P. — You shall, my dear ; but for this experiment 
we shall require the air-pump. I shall show you that ether 
may boil even when it is cold enough to freeze water ; 
and, what is still more extraordinary, that its boiling will 
cause the water to freeze. 

* There is a little instrument, called the pulse-glass, by means of which 
the effect of removing - the pressure of the atmosphere from a liquid, is ex- 
hibited in a very striking- manner. The instrument consists of a glass tube 
with a bulb at each end, the tube being bent at right angles where it joins 
the bulbs. One of the bulbs is about half filled with ether or spirit of wine; 
and, the air being first expelled, the tube is hermetically sealed. If the bulb 
containing the spirit be heated more than the other, the steam or vapor 
will escape from it, and pass into the cold bulb, where it will be condensed ; 
and in this way the whole spirit will, in a short time, pass from the one to the 
other. The atmospheric pressure being removed, it requires only the heat 
of the hand to make the spirit boil. — Am. Ed. 



56 BOILING. 

Harriet. — That will, indeed, be strange. 

Robert. — Yes, if ether boils with freezing cold, it 
will be curious. 

(Mr. Powell places the air-pump on the table. Under 
the receiver he puts a small glass full of ether, and on the 
top of the ether, and floating on its surface, a watch-glass 
containing a little water.) 

Mr. P. — Having put the ether and the water under 
the receiver, I will exhaust the air rapidly : now look 
at the ether. 

Frederick. — It seems to be boiling away as if there 
were a fire under it. 

Robert. — Yes, indeed it is; I cannot help thinking 
it must be hot. 

Mr. P. — I will let in the air, and take off the 
receiver, that you may feel. Now, Robert, put your 
finger into the ether, and let us know if it be hot. 

Robert. — (Touching the ether.) No, it feels quite cold. 

Frederick. — And the water in the watch-glass 
is frozen into a mass of ice ! Do explain the cause 
of this, father. 

Mr. P. — -After what I have told you respecting the 
effect of the pressure of the atmosphere on the boiling 
of fluids, I need not, I suppose, say more about the 
cause of the ether boiling, than that its vapor is so 
elastic as to overcome the pressure of the atmosphere 
when heated to only 90° ; and that, when the pressure 
of the atmosphere is removed, the ether becomes very 
rapidly converted into vapor, even when its temperature 
is 76° lower than the freezing point. 

Frederick. — But what made the water freeze while 
the ether was boiling? 

Mr. P. — You must understand, in the first place, that 
steam and vapor contain a very large quantity of heat, 



BOILING. 57 

which they abstract from liquids when boiling ; and it is 
owing to the quantity of heat they thus carry away, that 
liquids cannot be heated above their boiling points. 
This is the cause why water, when placed in an open 
sauce-pan over a hot fire, cannot become hotter than 
212°, though the fire itself is perhaps upwards of 
2000° ; for the steam deprives the water of the heat 
as fast as it is communicated. Thus, in the case of 
the ether, when the pressure of the air was removed, 
the vapor, into which it was rapidly converted, drew 
the heat from the ether, and from the water floating on 
its surface, so quickly, that the temperature of the 
water was reduced below 32°, and it became frozen. 

Frederick. — Do all vapors, whether cold or hot, 
take heat from bodies in the same way 1 

Mr. P. — Yes, my dear ; and there is reason to believe, 
that when vapors rise from a comparatively cold surface, 
they contain really more heat than when the liquids are 
boiling. It is owing to this absorption of heat by vapor, 
that we feel such a sensation of cold from wet clothes; 
as the evaporation of the moisture draws the heat from 
our bodies, which is carried off by the vapor. 

Harriet. — What is the reason, that people do not 
take cold when wet with sea water so soon as with 
fresh water l 

Mr. P. — The salt contained in sea water prevents 
the liquid from evaporating so quickly ; therefore the 
sensation of cold is not so great, because the heat is 
drawn from the body more gradually. Spirits, on the 
contrary, evaporate much more rapidly than water, and 
the cold produced by their evaporation is, consequently, 
much greater. If, for instance, you drop ether on the 
back of your hand, it feels extremely cold; and the 
effect is increased by doing so in the full heat of the 



.58 BOILING. 

sun, as the evaporation then beeomes quicker ; indeed, 
so rapidly is the heat abstracted, that small animals 
may be frozen to death, in this manner, under the heat of 
a summer's sun. 

Frederick. — Thank you, father, for these interesting 
explanations, which I hope I shall remember ; but I should 
like to know something more about the nature of steam, 
which is now made to do every thing. Can you tell us what 
makes it so strong as to be able to do the work of horses ? 

Mr. P. — I will endeavor, my dear, to give you some 
idea of the subject to-morrow. 



QUESTIONS. 

1. Can water be made to boil without the direct application 
of heat ? 

2. By what experiment may this be shown ? 

3. Describe the process of boiling, and give the reasons. 

4. What causes the ebullition of water ? 

5. What is the boiling point of water ? 

6. Why' does not water boil below 212° ? 

7. How does the pressure of the atmosphere prevent the steam 
from rising ? 

8. What is the force of the vapor of water at 32° ? 

9. What is its force at 212° ? 

10. Why cannot water be made hotter than 212°, under the 
pressure of the atmosphere ? 

11. At what degree will water boil in a vacuum? 

12. How many degrees are added to the boiling points of liquids, 
in general, by the pressure of the atmosphere ? 

13. Is boiling water as hot on a mountain as in a valley ? — Why 
not? 

14. Explain the experiment, by which hot water is made to boil 
by the application of cold. 



BOILING. 59 

15. Describe the experiment by which ether is made to boil and 
water to freeze at the same time. 

16. How do you explain this experiment ? 

17. Is boiling ether hot ? 

18. Why are damp clothes cold ? 

19. In what way can small animals be frozen, in the heat of 
summer ? 

20. What is the reason that people are not so liable to" take cold, 
when wet with salt water, as with fresh ? 



60 STEAM AND VAPOR. 



CONVERSATION VII 



STEAM AND VAPOR. 



Frederick. — Is the steam, we see coming out of the 
spout of the kettle, the same kind as that which works 
steam engines ? 

Mr. P. — You are mistaken, my dear, in supposing you 
see the steam; for steam is invisible like air* and most 
other elastic fluids; but the steam in the kettle, which you 
cannot see, is the same as that employed in working 
engines. 

Robert. — Not see the steam ! Why, look, father ! it is 
coming from the kettle spout most furiously. I am sure 
I see it. 

* The air is generally regarded as an invisible, colorless fluid j but this 
does not seem to be strictly true. The azure or blue appearance of the 
firmament, is owing to the mass of air through which we look ; it is, in a 
word, the color of the air. So, too, distant mountains appear blue, not 
because they are blue, but because we see them through a blue medium. 

The air in a room, or about us when we are abroad, is invisible on ac- 
count of its rarity, and the consequent feebleness with which it reflects its 
color. We can see it only when we look through large masses. 

This may be illustrated by a reference to various liquids, which appear 
colorless when viewed in small quantities. Sherry wine, for instance, has 
the appearance of water, in a very small glass tube. The water of the ocean 
is of a greenish hue ; but a portion of this green water, taken up in a tumbler, 
seems perfectly limpid and colorless. See Lardner's Pneumatics, Chapter 
II.— Am. Ed. 



STEAM AND VAPOR. @l 

Harrikt. — And so do I. 

Mr. P. — You see a stream of something, resembling a 
thick mist, issuing from the kettle ; but that is not steam : 
that mist consists of minute particles of water, into which 
the steam is condensed as soon as it comes in contact 
with the air. If you look close to the spout, you will per- 
ceive there is a space of about half an inch, between the 
spout and what you call steam, in which you see nothing: 
that is steam. 

Frederick. — Yes, I see a vacancy between the spout 
and the steam ; yet there must be something there, for the 
misty steam seems to come from it. 

Mr. P. — You observe, that what you call " misty steam" 
becomes less perceptible as it approaches the spout, be- 
cause the condensation of the steam does not take place 
all at once. Near the spout only a partial condensation 
occurs ; but as the steam issues farther into the air, it 
expands, and, a larger surface being thus exposed, it is 
completely condensed. 

Frederick. — What is the cause of steam condensing so 
quickly ? 

Mr. P. — A large portion of heat is always required to 
prevent vapor from condensing; and the elasticity, the 
invisibility, and the great bulk of vapors, compared with 
liquids, are supposed to be owing to the large quantity of 
heat they contain. Thus, steam of the temperature of 
212°, which equals the elasticity of the air, is rapidly con- 
densed into water when exposed to a lower temperature, 
because the heat necessary to preserve it in that state of 
elasticity is then taken from it. The vapor of spirits, 
being more elastic, (that is, having a greater attraction for 
heat,) is not so easily condensed as steam ; and ether, 
indeed, will not remain in a liquid state at a temperature 
higher than 90° ; therefore, in hot climates, ether can 
6 



62 STEAM AND VAPOR. 

exist only as an invisible vapor, unless confined under 
pressure. 

Frederick. — What is the bulk of steam compared with 
water ? 

Mr. P. — At the temperature of boiling water, steam oc- 
cupies 1S00 times the space of the water from which it is 
formed : therefore, a pint of water, when converted in^o 
steam, would fill 225 gallons. 

Frederick. — But when steam is converted into water 
again, what becomes of the space it filled? 

Mr. P. — If the steam, when confined in an air-tight ves- 
sel, be completely condensed, the space it occupied is left 
vacant ; and unless the vessel be strong, the pressure of the 
atmosphere on the outside would press in the sides.* I 
can show this to be the case by an easy experiment. 

Harriet. — I should like to see it, very much. 

Mr. P. — Into this moist bladder I put a tea-spoonful of 
ether, and after squeezing out the air I will tie it up. Now 
notice what takes place when I put it into hot water. 
( Mr. Powell holds the bladder in a basin of hot icater, 
when it almost immediately becomes inflated.) 

Harriet. — It has blown the bladder out. 

Mr. P. — Yes ; the ether is converted into vapor by the 
heat, and has filled the bladder. I will now put it into cold 
water, which will condense it into a liquid again. (Mr. 
Powell inunerses the bladder in cold icater.) 

* The pressure of the atmosphere is fifteen pounds on every square inch 5 
which, on a boiler twenty feet long and four feet in diameter, amounts to 
more than two hundred and sixty tons. Should the steam in such a boiler 
become suddenly condensed, it would be crushed, were it not of sufficient 
strength to resist this pressure. An accident of this kind is called a collapse. 
As a protection as^unst this danger, the boiler is sometimes provided with a 
safety-valve, opening inwards 3 that the air may enter as the steam becomes 
condensed. — Am. Ed. 



STEAM AND VAPOR. 



63 



Frederick. — Look, how the bladder shrinks ! it seems 
as if it were alive : what makes it do so? 

Mr. P. — The ether, as it condenses, leaves a vacuum in 
the bladder, and the pressure of the atmosphere forces its 
sides together. Any vessel, if not made strong enough to 
resist the pressure, would do the same. 

Robert. — Would water produce the same effect as the 
ether ? 

Mr. P. — Yes, exactly, if it were heated to the boiling 
point. I used ether because it is changed into vapor by a 
lower degree of heat, and therefore the experiment could 
be more readily performed. 

Frederick. — It seems a very curious'thing that the bulk 
of steam should be all at once lessened to a mere nothing 
by cold. Is any use made of this property in the steam 
engine ? 

Mr. P. — The whole power produced by many steam 
engines depends entirely upon it. In the first engine 

made by the celebrated Mr. 

Fig. I. Watt, all the power gained 

depended on the pressure 

of the atmosphere upon a 

piston moving in a cylinder, 

which was forced down by 

the atmospheric pressure 

when the steam underneath 

was condensed. But I 

shall make the subject 

more clear by this drawing. 

In this section, a b c d 

represents a large cylinder, 

one foot or more in diameter, with a movable piston, p, 

fitting tight to the sides of the cylinder. At the bottom 

there are two openings, b and c ; one of which, c, commu- 




^ 



64 STEAM AND VAPOR. 

nicates with a vessel containing cold water, to act as a 
condenser, and the other, b, communicates with the boiler. 
Now, supposing the cylinder to be filled with steam, and 
the stop-cock e, communicating with the boiler, to be shut, 
and the stop-cock g to be opened, the steam will rush 
along the pipe communicating with the condenser, and be 
immediately condensed into water, thereby producing a 
vacuum under the piston, which will then be forced down 
to the bottom of the cylinder by the pressure of the 
atmosphere. 

Robert. — How can the piston be raised up again ? 

Mr. P. — As soon as it arrives at the bottom, the stop- 
cocks are turned, so as to cut off the communication with 
the condenser, and to let the steam in again from the 
boiler. The pressure on the piston being thus removed, 
by the elasticity of the steam underneath, it is raised to 
the top by a heavy weight placed at the end of a large 
beam. The motion of the piston and beam, up and down, 
is thus continued as long as the steam is supplied in proper 
quantities, and the water in the condenser continues cold 
enough to condense it. 

Frederick. — I don't remember to have seen any weights 
at the end of the beams of those steam engines I have 
seen. 

Mr. P. — No, my dear ; for in Mr. Watt's improved 
engines he introduced the steam at the top of the piston, as 
well as underneath, and in that manner produced a vacu- 
um above as well as below it ; and the piston in these 
engines is forced both up and down by the pressure of the 
steam acting against a vacuum. 

Frederick. — Are all steam engines formed upon the 
principle of making a vacuum by condensing the steam ? 

Mr. P. — No, they are not. The engines I have de- 
scribed are called condensing engines, because their action 



STEAM AND VAPOR. 65 

depends upon the condensation of the steam. The 
engines now most in use are made to act upon a very 
different principle, depending upon another property of 
steam, viz. its elasticity. 

Harriet. — -What do you mean by saying steam is 
elastic ? 

Mr. P. — By elastic, I mean the power steam possesses 
of regaining its original bulk after having been compressed 
into a smaller compass ; and the force it exerts in trying 
to regain its natural degree of expansion, I call its elastic 
power. It is in the application of this force that the prin- 
ciple of the high-pressure steam engines consists. 

Frederick. — How can steam be compressed so as to 
exert the force that is wanted? 

Mr. P. — By heat. When water is heated to 240°, the 
steam exerts an additional force of about 15 lbs. on every 
square inch, being equal to the pressure of another 
atmosphere. 

Robert. — But I thought you said, father, that water 
could not be heated above the boiling point ; then how can 
it be made so hot as 240°, which is 28° above boiling? 

Mr. P. — I said it could not be heated above the boiling 
point if the steam were allowed to escape; but, if we con- 
fine the steam, and thus prevent the heat from flying off, 
water may be made as hot as any other substance, provided 
the vessel be strong enough. 

Harriet. — Why need it be so very strong? 

Mr. P. — Because the force of steam at very high tem- 
peratures becomes almost irresistible. It is ascertained that 
at 439° steam exerts a force of 375 lbs. on every square 
inch ; and as the heat is increased, the strength of the 
steam becomes greater. Mr. Perkins, indeed, is stated to 
have heated water red hot, and to have generated steam 
that acted with a force of 1500 lbs. per square inch. If a 
6* 



66 STEAM AND VAPOR. 

vessel the size of a common tea-kettle contained steam of 
this power, the pressure on its whole surface would exceed 
400,000 lbs. 

Frederick. — Can you show us any experiment, father, 
that will exhibit this elastic power of steam ? 

Mr. P. — Most experiments with high pressure steam are 
attended with some danger, and therefore ought to be ex- 
hibited with great caution. I think I can, however, give 
you some idea of the force of steam by a very simple appa- 
ratus. A piece of copper gas-tube, that has one end 
closed, and a bladder tied to the other end, will answer the 
purpose. (Mr. Powell produces apiece of gas-tube about 
a foot in length and a quarter of an inch diameter, brazed 
at one end, into which he pours a tea-spoonfid of water, and 
then ties a bladder very firmly on to the other end.) I will 
now place the end of the tube, where the water is contained, 
on the fire in a perpendicular position, that it may become 
hot. Stand at a distance from it, my children, lest the 
water should be thrown out and scald you. 

Harriet. — Papa, what will it do? Is it likely to 
hurt us? 

Frederick. — Look ! look ! the bladder is swelling out 
as if it were blown. 

Mr. P. — The steam is beginning to form, and is filling 
the bladder. You see it is now about to burst by the force 
of the confined steam. ( The bladder bursts with a loud 
explosion.) 

Harriet. — Oh dear ! what a tremendous noise ! I am 
almost stunned. 

Mr. P. — The bladder, you perceive, is burst at last, the 
pressure having become too great for the strength of the 
bladder to resist it. 

Frederick. — Is it in this way that accidents happen in 
steam engines? 



STEAM AND VAPOR. 67 

Mr. P. — Yes, my dear ; the bladder in this experiment 
might represent the boiler of a steam engine, in which the 
steam was forced up beyond the power of the boiler to 
bear it.* 

Robert. — If accidents are so likely to happen with 
high-pressure steam engines, why are they ever used, when 
the other kind would do as well ? 

Mr. P. — High-pressure engines possess many advan- 
tages which render them preferable, in particular circum- 
stances, to condensing ones. In the first place, they are 
cheaper, as the whole apparatus of the condenser is spared. 
In the next place, it often happens that it is impossible to 
procure so large a supply of cold water as is required to 
condense the steam, — in moving engines on rail-roads, 
more especially ; and, in the third place, high-pressure 
steam engines require less fuel. 

Frederick. — What is the reason of that, father I 

Mr. P. — I explained to you, the other day, that water 
requires to be heated to 212°, before the elastic force of 
steam is sufficient to balance the pressure of the atmos- 
phere ; but having attained that point, it only requires 28° 
of additional heat to double its power ; 25° more to treble 
it, and 18° additional to quadruple it; each succeeding 
atmosphere of pressure being gained by a lower proportion 
of heat. At a temperature of 367°, the force of steam is 
increased ten times. Now, if the water, before being 
heated, was at the temperature of 57°, it must require 155° 
of heat to raise it to the boiling point ; the whole of which 

* To guard against accidents of this sort, boilers are furnished with 
safety-valves, opening outwards, by which the steam may escape, when the 
pressure becomes too great. There is no danger that a boiler will burst, 
while it is kept in good order, if it have a sufficient and proper safety-valve. 
—Am. Ed. 



68 



STEAM AND VAPOR. 



heat will, after all, only produce steam of force sufficient 
to balance the weight of the atmosphere. 

Frederick. — Yes, I think I see how it is now ; 28° of 
heat added after 212° are equal in effect to 155° before; 
for steam formed at 212° will only have one half the 
strength of the steam from water heated to 240°. 

Mr. P. — Yes, Frederick, you are right. Thus you per- 
ceive that 155° added to 212°, will produce steam possess- 
ing ten times the power that the same quantity of heat 
would produce, if employed in raising the water merely to 
the boiling point. 

Robert. — You have told us, father, that steam and 
vapor contain a great deal of heat; how can this be 
known ? 

Mr. P. — It can be proved to be the case by many 
experiments; but one will be enough for the present. 

I will put into this 
il S- -W- retort a quarter of a 

pint of water, and in- 
sert the end of the 
tube into a basin con- 
taining a gallon of 
water, at the tempera- 
ture of 50°, and then 
apply a lamp to the 
retort till the water 
boils away. (Mr. Powell arranges the apparatus in the 
manner described, and exhibited in the annexed icood-cut.) 
In the mean time, let us see how much the temperature 
of one gallon of water at 50° is raised by adding to it a 
quarter of a pint of boiling water. [Mr. Powell mixes a 
quarter of a pint of boiling water icith a gallon of cold.) 
Frederick, observe how much the thermometer rises. 




STEAM AND VAPOR. 69 

Frederick. — (Holding the bulb in the water.) It has 
risen full nine degrees. 

Mr. P. — Very well : now, if the steam from the quarter 
of a pint of water in the retort contain only the same 
quantity of heat as boiling water, the temperature of the 
water in which it is condensed will be raised only nine 
degrees. 

Robert. — I should not think it would be so much. 

Mr. P. — Frederick, let us know how the thermometer 
decides the question ; for the water is just boiled away. 

Frederick. — (Putting the thermometer into the water.) 
The water is quite hot ; and see, the thermometer has risen 
to 118°, which is 63° higher than the water was before. 

Mr. P. — That is as I expected it would be ; for steam 
contains about 1000° more of heat than the same weight 
of boiling water. 

Robert. — I do not understand how that can be ; for 
steam cannot be hotter than boiling water. 

Mr. P. — An equal bulk of steam does not contain nearly 
so much heat as an equal bulk of boiling water ; but you 
must bear in mind, that one pint of water would fill 1800 
pints when converted into steam ; and the heat diffused 
through this large space, when condensed into one pint, is 
seven times greater than the heat in the same weight of 
boiling water. Steam or vapor, it is supposed, always con- 
tains the same quantity of heat, whether it be formed at 
a temperature of 60° or 600°. 

Robert. — What ! do you mean, father, that when steam 
is made three times as hot as boiling water, it does not 
contain more heat than common steam in a kettle 1 

Mr. P. — I mean that the same weight of steam does 
not ; because the more steam is heated, the more it is com- 
pressed, and, consequently, it weighs heavier; and a 



70 STEAM AND VAPOR. 

smaller quantity of high-pressure steam will therefore be 
condensed into as much water as a larger quantity of 
common steam. 

Robert. — Ay, I think I understand it now : what the 
steam gains in heat it loses in bulk ; is not that it ? 

Mr. P. — Just so. This has been rather a puzzling 
subject ; but I hope I have succeeded in making you un- 
derstand the general properties of steam. A more particular 
consideration of the different applications of steam, as a 
moving power, does not come within the scope of our 
present consideration ; though I may, at a future time, 
take an opportunity of explaining that subject to you 
more fully. 



QUESTIONS. 



1. What is steam? 

2. Is the mist which rises from hot water steam ? 

3. What do you understand by an elastic fluid ? 

4. What makes steam elastic and invisible ? 

5. At what temperature is steam as elastic as air ? 

6. What is the bulk of steam compared with that of water ? 

7. How can steam be condensed to a liquid ? 

8. By what experiment is this illustrated ? 

9. When is the boiler of a steam engine said to collapse ? 

10. How is this accident caused ? 

11. How many kinds of steam engines are in use ? 

12. Upon what did the oower of the first condensing or low- 
pressure engines depend ? 

13. Explain Figure 1. 

14. Is the action of all condensing engines produced by atmos- 
pheric pressure ? 

15. Who introduced a great improvement in steam engines ? 



STEAM AND VAPOR. 71 

16. How is the power of high-pressure or non-condensing engines 
produced ? 

17. How can steam be made to exert any required elastic force ? 

18. What is the force of steam at 240° ?— at 439° ? 

19. What causes the boilers of engines to burst ? 

20. What are some of the advantages of high-pressure engines ? 

21. Why do they require less fuel? 
"22. Explain Figure 2. 

23. What fact is proved by this experiment ? 

24. How much more heat does steam contain than an equal 
tcelght of boiling water ? 

25. In equal bulks of steam and boiling water, which contains the 
greater quantity of heat ? 



72 CLOUDS, FOGS, 



CONVERSATION VIII 



CLOUDS, FOGS, AND DEW. 



(A thick Fog.) 

Frederick. — What a very foggy morning this is ! I 
can scarcely see across the road. 

Harriet. — I should like very much to know what it is 
that makes these disagreeable fogs. 

Mr. P. — If you understood what I told you respecting 
the nature of steam, I think it will not require much time 
to explain the nature and cause of fogs. In our conversa- 
tion upon steam and vapor, we considered only the property 
of that vapor which is given out by liquids in a boiling 
state ; but fluids, even at their coldest temperatures, are 
continually, though slowly, being changed into vapor. 
This slow process is called evaporation. 

Robert. — But I thought vapors contained a great quan- 
tity of heat ; then how can vapor be made in cold weather ? 

Mr. P. — You cannot, surely, have forgotten what I so 
recently explained to you respecting the heat contained in 
the atmosphere, even at the lowest temperatures. You 
must remember that, even in the coldest weather we ever 
experience in this country, the air is quite hot, compared 
with the temperature near the poles. It is from this source, 
and from the heat of the earth, therefore, that vapor derives 



AND DEW, 73 

its heat. The process is always going on, though very 
slowly, compared with the vapor formed at boiling heat. 

Frederick. — I suppose that evaporation goes on faster in 
hot than in cold weather. 

Mr. P. — You are right. In summer, the quantity of 
water evaporated from one acre of land, after heavy rain, is 
estimated at 1900 gallons in twelve hours. Even when 
apparently quite dry, the ground is continually parting with 
vapor to the atmosphere, though not in such large 
quantities. 

Harriet, — But what becomes of the vapor ? 

Mr. P. — As vapor is lighter than the air near the surface 
of the earth, it ascends into the colder regions of the upper 
air, till, being deprived of a portion of its heat, it is partially 
condensed into water, in the same manner as you observe 
the steam issuing from the kettle is condensed into a mist. 
This condensed vapor forms clouds. 

Robert, — If the clouds consist of water, why do they 
float so high in the air, instead of falling directly to the 
ground, as water would do ? 

Mr. P.— Each particle of water condensed from the 
vapor, is so . minute as not to be separately visible to the 
naked eye. Its weight is, therefore, not sufficient to 
counterbalance the resistance of the air to its descent; and 
when large quantities of these minute particles are col- 
lected together, as in clouds, the extended surface they 
present to the air helps to sustain them. Besides, the 
whole mass being kept distended by mixture with the air, 
the weight scarcely exceeds that of the surrounding 
atmosphere, and must be absolutely lighter than the air 
near the earth's surface. 

Frederick. — Are fogs, then, any thing like clouds 1 

Mr. P. — Yes, Frederick; they are formed in the 
same manner. The only difference between them is, 
7 



74 CLOUDS,. FOGS, 

that the vapor which forms a fog, is condensed before 
it can ascend from the earth. 

Robert. — But why is vapor sometimes condensed 
into fog, and sometimes into clouds ? Why is it not always 
the same, either all fog or all clouds? 

Mr. P. — That entirely depends upon the temperature 
of the air, as compared with that of the earth, or water, 
from which the vapor rises. If the surface of the 
earth be hotter than, the surrounding air, the vapor, in 
this case, obtains the greater portion of its heat from 
the earth, or water, and, rising into the colder air, it is 
almost immediately condensed before it can ascend. 
If, on the contrary, the air be warmer than water, the 
vapor rises uncondensed. The atmosphere, in the latter 
case, is clear, and, in the former, foggy. 

Robert. — But if we have fogs because the air is 
colder than the earth, the weather ought always to be 
foggy in a frost, which it is not. 

Mr. P. — I am not surprised at your wondering why 
fogs are not produced by frost. There are two reasons 
why the atmosphere is generally clear in a hard frost. 
In the first place, the water and the moisture on the 
earth's surface being then frozen, a much less quantity 
of vapor is formed ; and, in the second place, the air 
condenses the vapor as soon as formed, and it is frozen 
before it can rise from the earth, and produces what is 
called a hoarfrost. 

Harriet. — What is it that makes the grass so wet 
on summer mornings, when there is no fog ? 

Mr. P. — It is owing to the comparative coldness of 
the grass. The earth, having become heated by the sun 
during the day, is hotter than the air after the sun is 
gone down ; and part of the vapor is, therefore, con- 
densed before it can ascend, and forms deio. It has been 



AND DEW. 75 

ascertained by the late Dr. Wells, who made a series 
of experiments with a view to explain the phenomena 
of dew, that the air close to the earth is several degrees 
colder during the formation of dew, than it is four or 
five feet from the ground. The difference he supposed 
to be owing to the radiation of heat from the surface 
of the earth ; and it is a remarkable fact, that, on clear 
nights, when there are no clouds to reflect the heat 
again to the earth, this difference of temperature is most 
observable ; and on these nights the greatest quantity 
of dew is deposited. On cloudy and windy nights, there 
is scarcely any dew formed. 

Frederick. — What is the reason, father, that dew will 
not fall on highly-polished steel ? 

Mr. P. — Dr. Wells found that polished metals, and 
all substances that radiate heat very imperfectly, are 
warmer on clear nights than those from whose surfaces 
heat is radiated more rapidly ; and, in consequence of 
their being warmer, less dew will be formed upon them. 
The circumstance that dew is deposited in different 
quantities upon different substances, and that those on 
which it collects are the best radiators of heat, strongly 
confirms the radiating theory. 

Harriet. — I should suppose, then, that grass sent 
out a great deal of heat in this way ; for it often seems 
quite drenched with dew. 

Mr. P. — Yes, it does, Harriet. Gravel, on the 
contrary, radiates comparatively very little heat, which 
is the cause of the walks in the garden being dry when 
the grass is not fit to walk upon. Wool, cotton, and 
all fibrous substances, are found to radiate a large quantity 
of heat, and to become the coldest when exposed on 
a clear night. But, whatever may be the cause of one 
substance becoming cooler than another, the effect is 



76 CLOUDS,. FOGS, 

the same as regards the formation of dew ; for the 
vapor will not be condensed, unless the air, or the 
substances on which the dew is deposited, is colder 
than vapor. 

Frederick. — Then, so long as the air is hotter than 
the earth, there will be no fogs ? 

Mr. P. — No, there will not. In spring, before the 
earth has received much heat from the sun, fogs are not 
nearly so frequent as in the autumn, at which time the 
earth is, at night, generally warmer than the air. You 
have, I dare say, observed the canal, on a frosty Sep- 
tember morning, looking as if it were almost boiling. 
This appearance is owing to the water being much 
warmer than the air; and the vapor, being therefore 
condensed as it rises from the surface, presents the 
appearance of hot, steaming water. 

Harriet. — But why are fogs so much more common 
in valleys than on hills ? 

Mr. P. — Valleys generally contain more moisture than 
hills ; therefore a larger quantity of vapor is formed there. 
Besides, the tops of hills are more exposed to the wind, 
which dissipates the fog. 

Harriet. — How does the wind clear away fogs 1 

Mr. P. — It does so in two ways ; first, by mechan- 
ically blowing the condensed vapor away ; secondly, a 
constantly fresh current of air being brought into contact 
with the minute particles of water composing the fog, 
they are dissolved again into vapor. 

Frederick. — Is there always vapor in the air ? 

Mr. P. — Yes, Frederick; in the most brilliant day 
of summer, the air is more charged with vapor than in 
the foggy days of November; but the heat keeps the 
moisture in a state of vapor, and it is, therefore,^ invisible. 

Robert. — How can it be known, then? 



AND DEW. 77 

Mr. P — The existence of vapor may be easily detected. 
For instance : if, on a fine summer's day, an empty 
glass be brought out of a cold cellar into the open air, 
it will instantly be covered with mist; for the vapor in 
the air will be condensed by the cold glass, and the 
moisture will adhere to its sides. In the same manner, 
when a thaw suddenly succeeds a hard frost, the walls 
of a house will run down with wet. This phenomenon 
depends upon the same cause ; for, in consequence of 
the thickness of the walls, they cannot change their 
temperature so quickly as the air, and will remain for 
some days colder than the atmosphere. By this means, 
the walls condense the invisible vapor, and the moisture 
adheres to them, and runs down in streams. 

Frederick. — Yes, I have noticed the walls of our 
house do so in a thaw, and I thought it was owing to 
the house being damp. 

Mr. P. — Many a house gets a bad character from 
the same cause, very undeservedly. The thicker the 
walls, the longer will the moisture adhere to them, 
provided the previous frost has been long enough to 
penetrate the bricks. 

Frederick. — I suppose that, in the same manner you 
have explained the cause of fogs, you account for our 
seeing the breath of people in cold weather. 

Mr. P. — You are right, Frederick. The vapor, as it 
issues from the mouth, becomes condensed by the cold, 
and is, therefore, rendered visible. 

Harriet. — It is very disagreeable to be following 
people in a frost, and to have all their breath blowing 
into one's face. That makes me dislike walking in the 
streets in a frost. 

Robert. — Or to come near a horse that is blowing 
clouds at you through his nostrils— eh, Harriet 1 
7* 



78 CLOUDS, FOGS, 

Harriet. — Oh, shocking ! 

Mr. P. — It is all fancy, Harriet. The same thing 
takes place in the most beautiful day in summer ; but as 
the vapor is then invisible, you do not think of any 
annoyance. This> and numberless other annoyances of 
the same kind, are disagreeable only in idea. You should 
endeavor, as much as possible, to overcome your aversion 
to such trifles; if not, you will continually be made 
uncomfortable, and be disgusted with matters that ought 
not to give you the slightest uneasiness. 



QUESTIONS. 

1. "What do you understand by evaporation ? 

2. How is vapor made in cold weather ? 

3. In what weather does evaporation go on most rapidly ? 

4. What fact is stated as to the quantity of water evaporated in 
the summer ? 

5. How are clouds formed ? 

6. Why do the clouds float in the air ? 

7. Do fogs resemble clouds ? — In what do they differ ? 

8. Why does vapor sometimes form clouds, and at other times 
fogs? 

9. Why are there no fogs in frosty weather ? 

10. How is frost produced ? 

11. How is dew formed ? 

12. Is there generally more dew on a calm and clear night than 
when it is windy and cloudy ? 

13. In what way do you account for this ? 

14. Why does not dew form freely on polished metals ? 

15. Why are gravel walks less affected by dew than grass ? 

16. At what season of the year are fogs most frequent ? — How do 
you explain this fact ? 

17. Why are fogs more common in valleys than on hills ? 



AxND DEW. 79 

18. What effect has wind upon fogs ? 

19. At what season of the year does the air contain the greatest 
quantity of vapor ? 

20. How do you account for the moisture which settles on a 
pitcher of water, decanter, &c, in the summer? 

21. Why is it that the walls of houses are sometimes covered 
with moisture ? 

22. What is the reason that we can see the breath of people and 
animals in cold weather and not in warm ? 

23. Why is there less frost on a windy than on a calm night ? 



80 RAIN, SNOW, AND HAIL. 



CONVERSATION IX 

RAIN, SNOW, AND HAIL. 



Mr. P. — It is raining so very heavily this morning, 
that I think we cannot do better, my children, than stay 
at home, and endeavor to explain what causes rain to fall. 

Robert. — I think there can be no great difficulty in 
finding out that, however ; for every body knows that 
rain comes from the clouds. 

Mr. P. — Then you can perhaps tell us why the clouds 
sometimes rain, and at other times do not ; and why the 
rain descends in drops of such equal size, instead of 
coming down in masses. 

Robert. — No, I cannot tell that exactly ; but I know 
that it is the clouds that make the rain. 

Mr. P. — The rain comes from the clouds, no doubt ; 
but the manner in which rain is made, has puzzled 
older heads than yours, Robert ; and we cannot now 
speak positively as to the cause of rain. 

Frederick. — Is it not owing to the condensation of 
the vapor 1 

Mr. P. — Yes, it is owing to the condensation of the 
vapor ; but you must recollect that all clouds consist 
of condensed vapor, yet all clouds do not rain. 

Frederick. — Then what is supposed to be the 
cause, father? 



RAIN, SNOW, AND HAIL. 81 

Mr. P. — The immediate cause of rain is owing to 
the partially condensed vapor of the clouds having been 
rendered more dense than before. In this more con- 
densed state, the clouds resemble what are termed mists, 
in which the particles of water are separately visible, 
though extremely small, and possess sufficient weight 
to fall to the ground. 

Robert. — But what can make the clouds more dense 
at one time than at another 1 

Mr. P. — There are many causes that would account 
for it. Suppose, for instance, a cloud to be continually 
receiving fresh supplies of vapor from the earth, which 
vapor becomes condensed on entering it. The additional 
vapor, when thus condensed, would unite with the minute 
particles of water forming the cloud, and, by continual 
addition, these particles would become minute but visible 
drops, like a mist. Again, if a cold current of air 
come in contact with vapor at a low elevation in the 
atmosphere, such vapor will be condensed into larger 
particles, than if it had been condensed higher in the 
air, where it would have been more expanded ; and a 
cloud formed near to the earth will, therefore, be more 
dense than when formed after the vapor has risen higher. 

Frederick. — Then I suppose, father, when the vapor 
is most rarefied, the drops of water into which it is 
condensed are the smallest. 

Mr. P.— They are. 

Robert. — But what makes vapor thinner at one 
time than another ? 

Mr. P. — The vapor is more or less rarefied, or, in 
other words, its elasticity is greater or less, according 
to the degree of heat at which it is formed. Thus, as 
I before told you, steam formed at a temperature of 240° 
has twice the elasticity of steam formed at 212°, the 



S'2 RAIN, SNOW, AND HAIL. 

boiling point ; and vapor formed on a hot day in summer 
possesses more elasticity than vapor produced on a cold 
day in November. 

Frederick. — Then does a gallon of the vapor formed 
in hot weather contain more water than a gallon of 
vapor formed on a cold day ? 

Mr. P. — Yes, that is the case. The cold vapor is 
more rarefied than the hot; and, therefore, when con- 
densed, the fog or cloud is not so thick.* Besides, the 
condensation of cold vapor must be carried on more 
slowly, for the difference in temperature between it and 
the air is not so great. 

Frederick. — Does the quick condensation of vapor, 
then, depend upon the difference in heat between the 
vapor and the air 1 

* A given portion of the atmosphere contains less vapor in cold weather 
than in warm, because the process of evaporation goes on more slowly ; 
and, therefore, "the cold vapor is more rarefied than the hot," if it be equally 
diffused through the air. But the elasticity, or the tendency to expand and 
become rarefied, is vastly greater in hot vapor than in cold. Hence it is, 
that the clouds usually float much higher in summer than in winter. 

In the spring of the year, before the ground has become warmed, the 
evaporation is, comparatively, sluggish ; the vapor produced, though thin, is 
imperfectly elastic, and the clouds are formed in the lower regions of the 
atmosphere. The drops of rain are, consequently, misty and small, as the}' 
reach the ground almost as soon as they arc formed. During the heat of 
summer, on the contrary, the vapors, being more elastic, ascend to a greater 
height, where the}' accumulate and form clouds. And the drops of rain are, 
at that season, large, because they pass through an atmosphere saturated 
with moisture, and increase in size in proportion to the height from which 
they descend. 

On these principles can be explained the seeming paradox, that hail- 
storms are more frequent in warm climates than in cold ; and that the largest 
masses of hail fall when the weather is hottest. The vapors formed under 
these circumstances, are very abundant, and have sufficient elastic force 
to ascend to a great height, where, meeting a cold body of air. they are 
congealed and precipitated in the form of hail. The round form of hail- 
stones is probably occasioned by a rotary motion as they descend. — ■ 
Am. Ed. 



RAIN, SNOW, AND HAIL. 83 

Mr. P. — Entirely so. Thus, you perceive that the hot 
steam from the kettle is instantly condensed as soon as it 
becomes exposed to the air ; whereas the vapor in the 
room, formed at the same temperature as the air, remains 
invisible. 

Robert. — But how can we be certain that there is any 
vapor in the room, when we cannot see it ? 

Mr. P. — I can, if you wish it, make the vapor visible. 

Robert. — Yes, I should like it by all means. 

Harriet. — And so should I, father. 

Mr. P. — [Producing a large empty phial.) You per- 
ceive this phial is apparently quite dry and clean. It is 
full of the same air as this room. I will cork it up ; and 
now, Robert, look at it. Do you suppose it contains any 
vapor ? 



Robert. — I think not, for it appears quite dry. 

Mr. P. — Take it into the cellar for a minute, and see 
what effect that has upon it. 

Robert. — {Returning with the phial after having taken 
it into the cellar.) Look, father! the bottle is all dim 
inside. 

Mr. P. — That is the vapor you could not see in this 
warm room, now condensed by the colder temperature of the 
cellar, and become visible. It is owing to the same cause 
that the vapor of the breath is condensed on the windows 
of the carriage. The vapor is invisible in the carriage, 
because the heat inside maintains it in a state of vapor ; 
but the windows, being cooled by the external air, condense 
the vapor as soon as it touches them. The frosty crystalli- 
zations on the inside of a window, may be explained in a 
similar way. The vapor in the room is condensed and 
frozen on coming in contact with the cold glass, and forms 
there a considerable thickness of ice, while in other parts 
of the room the vapor is not perceptible. 



84 RAIN, SNOW, AND HAIL. 

Harriet. — I remember, papa, that, when we went on 
board the steam-boat with you, last summer, a shower of 
small rain came on when the engineer let out the steam, 
though it was a very fine day : was that owing to the 
condensation of the steam by the air ? 

Mr. P. — I am very glad, Harriet, that you have re- 
minded me of the circumstance, as it is a complete illus- 
tration of the formation of rain. The steam, as it issued 
from the steam-pipe, was condensed immediately into a 
thick mist by the colder air ; and the minute particles of 
water first formed, having been enlarged by the conden- 
sation of more steam upon them, became sufficiently heavy 
to fall down in drops. 

Frederick. — Yes, I remember the shower very well ; 
but the drops were smaller than those in a common shower 
of rain. 

Mr. P. — The smallness of the drops was owing to the 
small quantity of condensed steam through which they had 
to fall, compared with the magnitude of clouds. If the 
mass of steam had been greater, the drops, in descending, 
would have collected more water round them : two or more 
drops would have united into one ; and they would have 
become equal to the largest drops of rain. 

Harriet. — Do the drops of rain unite in that way in 
falling from the clouds ? 

Mr. P. — We must conclude that they do ; for it is as- 
certained that the drops increase in size as they approach 
the ground. If we suppose the clouds to be of great 
thickness, we can easily imagine that, as soon as the 
smallest particles become large enough to descend, they 
will increase in size by uniting with other small drops in 
their descent. 

Frederick. — What effect has the wind in producing 
rain? 



RAIN, SNOW, AND HAIL. 85 

Mr. P. — The wind has very opposite effects upon the 
clouds, depending upon its temperature, its velocity, and 
the formation of the clouds themselves. A wind, warmer 
than the temperature of a cloud, will dissolve it into vapor ; 
and in this way you may often see clouds disappear in the 
air. If the wind be colder than the cloud, it will condense 
more vapor around it, and, by compressing the minute 
particles together, will assist their uniting into drops. If 
the mass of clouds be sufficiently great to resist the action 
of the wind, we may suppose that rain will be produced 
merely by the particles of condensed vapor in the middle 
of the clouds being compressed together. 

Frederick. — Then it appears that the wind is the 
principal cause of rain. 

Mr. P. — There is reason to believe that it is the chief 
agent ; but there may be other causes which produce the 
condensation of vapor, that we are, at present, unacquainted 
with. We may, however, take it for granted, that the 
vapor cannot be condensed, until it has parted with the 
heat that kept it in an invisible state. 

Harriet. — I suppose, then, that snow must be made 
from vapor that has lost all its heat 1 

Mr. P. — Not all, Harriet, but sufficient to reduce its 
temperature to the freezing point. Snow we must suppose 
to be formed by the freezing of the minute particles of 
moisture as they are condensed. The hoar frost upon the 
grass and trees is formed in the same manner, and, if 
closely examined, will be found to consist of a number of 
small crystals of ice.* 

* Hoar frost is dew, which freezes as it settles on the ground j and the 
manner in which dew is formed, has been explained. It is commonly ob- 
served, that there is less frost on a cloudy evening, than when the atmos- 
phere is clear. The reason of this probably is, that the earth, which has 
become heated during the day, radiates its heat during the night, especially 

8 



86 RAIN, SNOW, AND HAIL. 

Frederick. — What causes the difference between snow 
and hail ? 

Mr. P. — Snow is formed, as I have told you, by the 
freezing of the small particles of moisture before they are 
united into drops; hail, on the contrary, is first formed into 
drops, and is frozen as it descends to the ground. 

Frederick. — Yes, I understand now how it is; they 
are both frozen rain ; only snow is frozen in the clouds as 
it is made, and hail is nothing more than rain frozen as it 
comes down. 

Mr. P. — I am glad, Frederick, you have so clearly ex- 
plained the difference. The subject of this morning's con- 
versation is one of considerable difficulty and intricacy, if 
pursued in all its bearings ; but I trust I have given you a 
sufficient insight into the causes that may commonly pro- 
duce rain, to enable you to understand them, 

the earl}' part of it, towards the sky. If the atmosphere be clear, this heat 
passes off and is lost j and the earth soon becomes cold enough to con- 
dense the moisture near its surface. But if there be clouds in the air, they 
arrest the heat in its upward progress, and send it back again to the ground ; 
thus maintaining such a degree of warmth on the earth's surface, as prevents 
the dew and frost from forming. — Am. Ed. 



QUESTIONS. 

1. What is the cause of rain ? 

2. Why should clouds be more dense at one time than at 
another ? 

3. Upon what circumstance does the elasticity of vapor depend? 

4. Which is more dense, the vapor formed in cold weather or 
in hot ? 

5. Is there always vapor in the air? 



RAIN, SNOW, AND HAIL. 87 

6. How do you account for the frost on windows in cold 
weather ? 

7. How are drops of rain formed ? 

8. Why are the drops of rain small in spring? — Wny are they 
large in summer ? 

9. What effect has tha wind in producing rain ? 

10. How is snow formed ? — hoar frost ? 

11. What is the difference between snow and hail? 

12. Why does it more frequently hail in summer than in winter ? 
13~ Why are hail-stones larger in hot weather than in cold ? 

14. Why is there less frost on a cloudy than on a clear night ? 

15. How do you account for the incrustations of ice sometimes 
seen on trees? 



88 



FIRE. 



CONVERSATION X 



FIRE. 



Harriet. — You have told us, papa, a great deal about 
heat and cold ; but you have not yet said any thing 
about fire. 

Robert. — Every body knows that the fire in the grate 
comes from the coals. 

Mr. P. — Then every body is mistaken, Robert. The 
fire does not come from the coals ; they are only the means 
of bringing it into action ; the heat comes from the air. 

Harriet. — You cannot mean that the coals do not burn 
when put on the fire 1 

Mr. P. — I mean, that the heat, by which the coals are 
reduced to cinders and ashes, comes from the air, and not 
from the coals; and that they will not burn unless supplied 
with air. I will show you that air is necessary for the 
production of flame, by placing a lighted taper under a 
glass to exclude the air, when you will perceive that the 
light will shortly be extinguished. (M?\ 
Powell fastens a taper to a flat piece of 
cork, and lets it float on the surface of 
water contained in a saucer. After light- 
ing the taper, he covers it with a large 
glass, as represented in the annexed wood 
cut. 



Fig. III. 




FIRE. 89 

Harriet. — The taper is going out, papa. 

Mr. P. — Yes. It is now extinguished. 

Frederick. — And the water has risen higher in the 
glass than it was before. 

Mr. P. — Part of the air has been consumed by the 
taper, and the water is, therefore, forced higher to supply 
its place. You may learn from this experiment, that a 
candle will not burn without air, and that the air is con- 
sumed by the burning. The same holds good with regard 
to the fire ; for when the supply of air is stopped, as it can 
be in some stoves, the fire goes out. 

Robert. — But how can the air burn, father 1 

Mr. P. — Before I attempt to answer that question, it will 
be necessary to give you some idea of the composition of 
the air, and of the property that all bodies, and more 
especially airs and vapors, possess of containing heat in 
what is termed a latent, or hidden state. 

Harriet. — What do you mean, father, by saying that 
all things can hide heat ? 

Mr. P. — When any substance receives a quantity of 
heat without feeling hotter, the heat that it so receives is 
said to be latent ; because it is hidden from the senses, and 
its existence in the substance is not made apparent by the 
most sensitive thermometer. 

Robert. — Is it possible for any thing to be heated 
without becoming hotter 1 

Mr. P. — Yes, without becoming sensibly hotter, as I 
can readily convince you, if you will fetch me a piece of 
ice from the tub in the yard, that I saw frozen over this 
morning ; and bring at the same time a pint of the water 
from under the ice. 

Robert. — (Bringing the ice in a basin, and the water.) 
Here is the ice and the water, father ; if there be any heat 
in them, it is so well hidden that I can't find it out. 
8* 



90 FIRE. 

Mr. P. — But the thermometer can, as you know, 
Robert, from your experience the other day. We do not 
call the heat in the ice latent; for it is very sensibly 
warm, compared with a mixture of salt and snow. You 
see the ice and the water are of the same temperature ; 
the thermometer stands in each at 32°. I will weigh a 
pound of ice, and put it into a saucepan over the fire. 
When melted, it will be nearly equal in bulk to the cold 
water, which I will put into another saucepan over the 
same fire. We shall see presently which is the soonest 
heated. (Mr. Powell places the ice and the water on 
thejire in separate saucepans.) 

Harriet. — As they both get the same h^at from the 
fire, I suppose one will be as hot as the other. See, the 
ice is beginning to melt already. 

Robert. Yes, it is quite clear that that is getting warm. 

Mr. P. — Suppose, when the ice is nearly melted, you 
should find that the water is no hotter than the ice was 
at first ; what would you think had become of all the 
heat it is now receiving from the fire 1 

Robert. — I should think the water could play at 
hide-and-seek a great deal better than I can ; but that 
cannot be, you know, father. 

Mr. P. — Take both saucepans from the fire, for the 
ice is just melted : put the thermometer into the water 
dissolved from the ice, and let us know how much 
heat it has gained. 

Robert. — {Holding the thermometer in the water.) 
Why, I declare, the quicksilver has fallen nearly to 
freezing ! and the water really feels as cold as the ice did. 

Mr. P. — Now try the other water with the thermometer. 

Robert. — This seems to be nearly boiling, and the 
quicksilver is rising very fast. See, it is at 172°! 
How very strange this is ! 



FIRE. 91 

Frederick. — The water has, then, gained 140° of 
heat, while the melted ice does not appear to have gained 
any ; though the same quantity of heat must have 
entered both. 

Mr. P.— The 140° of heat gained by the melted 
ice are latent in the liquid ; that is, it has received 
140° of heat, in being converted from a solid into a 
liquid, without any sensible difference having been 
produced in its temperature. This quantity of heat 
seems necessary to keep water in a fluid state, and 
must be parted with before it can be again frozen. It 
can be proved, by other experiments, that ice absorbs, 
when melting, 140° of heat. For instance, if a pound 
of water at 172° be mixed with a pound of ice at 32°, 
the temperature of the mixture would be only 32° instead 
of 102°, as it would be if equal quantities of water of 
those temperatures were mixed together : thus, 140° 
must have become latent, or concealed, in the melted ice. 
It is owing to this absorption of heat by water that the 
process of freezing is so very slow ; for the water must 
first part with the heat that preserves it in a liquid state, 
before it can become solid. 

Frederick. — What other things, besides water, contain 
heat hidden in this manner 1 

Mr. P. — All solid bodies, on becoming fluid, either 
by melting or dissolving, absorb much more heat than 
the thermometer indicates ; and, in the melting of some 
metals, this quantity exceeds, by four or five times, the 
quantity absorbed by melting ice. When liquids are 
converted into airs or vapors, a still greater quantity of 
heat is rendered latent, than in the change of solids into 
liquids ; the quantity of latent heat generally increasing in 
proportion to the increase of the bulk of any body by 
the change. When water is converted into steam (which 



92 FIRE. 

occupies 1800 times the space of water), 1000° of heat 
become latent during the increase of bulk; that is, 
when one pint of water of 212° is changed into 1800 
pints of steam, which is also 212°, the 1800 pints of 
steam will contain 1000° more of heat than the pint of 
boiling water from which it was made, as I explained 
to you in our conversation on steam. If the same steam 
be again condensed into water, its latent heat will be 
discovered ; for it will give out as much heat as a pint 
of water would do if heated 1000° above boiling heat. 

Frederick. — Do you mean, father, that steam, which 
the thermometer will tell us is only as hot as boiling 
Avater, really is 1000° hotter ? 

. Mr. P. — The same weight of steam contains 1000° 
more of heat than the same weight of boiling water; 
though, as steam is 1800 times lighter than water, the 
same bulk of steam will not contain nearly so much 
heat as the same bulk of boiling water. As water, in 
melting from ice, absorbs 140°, and when, in expanding 
into steam, it absorbs 1000° more, steam we may con- 
sider as containing 1140° more of latent heat than ice. 
Steam, when formed at the temperature of boiling water, 
also contains 180° more of sensible heat than ice at 
32° ; therefore the latent heat and sensible heat, together, 
will make the same weight of steam 1320° hotter than 
the same weight of ice. If we could suddenly condense 
a pint of steam into a pound of ice, the 1320° of heat 
would be given out, and would be sufficient to produce 
the appearance of flame. Now, what would take place 
in the case of steam, if it could be condensed at once 
into ice, does actually take place with air during the 
burning of coals upon the fire. 

Frederick. — Air, then, I suppose, contains hidden 
heat, in the same manner as steam. 



FIRE. 93 

Mr. P. — Yes, my son, it does; though the quantity 
of latent heat in air has not been accurately ascertained. 
That it does contain heat, however, may easily be made 
apparent, by suddenly compressing air in a syringe with 
tinder placed at the bottom ; for the compression of the 
air will cause it to give out sufficient heat to set fire 
to the tinder. 

Frederick. — Does air become solid, then, father, 
by burning in the fire 1 

Mr. P. — A great part of the air that is consumed 
is converted into water, which is 600 times smaller in 
bulk than air ; and this condensation of air into water 
produces flame in fires.* 

Harriet. — What, papa! do you mean that it is 
water that makes the fire burn ? That is the strangest 
thing you have told us yet ; for water, we all know, 
puts out fire. 

Mr. P. — Yes, Harriet, it may seem very extraordinary, 
but it is the fact ; and though water extinguishes fire, 
no fire blazes without producing water. 

Robert. — Well, father, if fire and water are the 
same thing, I shall not dare to trust my senses any more. 

Mr. P. — Remember, Robert, they have failed you 
already in your attempts to distinguish hot from cold. 

Frederick. — But how is it that air can be changed 
into water? and in the fire too, where we always 
suppose there cannot possibly be any 1 

Mr. P. — To enable you to understand this interesting 
subject, it will be necessary to enter into an explanation 

* Lavoisier's theory of combustion has been adopted to explain the cause 
of fire, because, whatever difficulties may attend the general application of 
this theory, it appears to be admitted that the heat and light evolved in 
common combustion are principally produced by the liberation of latent 
heat. 



94 FIRE. 

of the nature and composition of the air of the atmos- 
phere : and as that will occupy some time, I will postpone 
the consideration of it until to-morrow. 



QUESTIONS. 

1 . Whence does a fire derive its heat ? 

2. Explain the experiment represented by Figure 3. 

3. What facts do you learn from this experiment ? 

4. What is latent heat ? 

5. Can any thing- receive heat without becoming sensibly hotter ? 
G. By what experiment is this proved ? 

7. How many degrees of heat are required to convert ice into 
water ? 

8. What other experiment shows the same fact ? 

9. Why is the process of freezing slow? 

10. What other bodies, besides water, contain latent heat ? 

11. How much heat becomes latent, when water is converted 
into steam ? 

12. How can it be proved that air contains latent heat ? 

13. What change does the air undergo in the process of burning? 

14. What principles are taught in this conversation ? 



FIRE. 95 



CONVERSATION XI 



FIRE (continued.) 



Frederick. — I am quite anxious, father, to know 
what the air can be made of, that it should be changed 
into water by the fire. 

Mr. P. — The air we breathe is composed of a mixture 
of two airs, or gases, called oxygen and nitrogen ; in 
the proportion of one fifth part of oxygen to four fifths 
of nitrogen. It is the oxygen gas in the air that causes 
combustibles to burn in it. If that gas be taken away, 
the most combustible body will not burn in the remaining 
nitrogen; and if any lighted substance be put into pure 
oxygen gas, it will burn with greatly increased brilliancy. 
I have prepared a jar of that gas, to enable you to 
see this effect. When I let this small lighted taper 
down into the jar, you will perceive how much more 
brilliant it will become. [Mr. Powell ties the taper 
to a piece of wire, and, when lighted, he introduces it 
into the jar of oxygen gas.) 

Harriet. — How very beautifully it burns ! It is almost 
too bright to look at. 

Frederick. — What is that mist I see round the jar, 
father, now that the taper is burnt out ? 

Mr. P. — That is the water formed by the burning 



96 FIRE. 

of the taper ; and in that water you see the oxygen gas 
condensed into a liquid. 

Robert. — Is it like common water 1 

Mr. P. — Yes, it is water in its purest form. All water 
is composed of two substances ; which we know, when 
separate, only in the form of gas : these substances are 
oxygen and hydrogen. Hydrogen gas is sixteen times 
lighter than atmospheric air ; yet, though so very light, 
it forms a great part of most combustibles, with which 
it is united in a solid form. Hydrogen and oxygen gases 
have a very strong attraction for each other, so that 
when heated to 800°, they instantly unite, and condense 
from the expanded form of gas into water, which is the 
product of their union. Water, therefore, is a mixture 
of hydrogen and oxygen. The proportions in which 
they unite to form water are, eight parts, by weight, of 
oxygen to one of hydrogen ; but as hydrogen gas is so 
much lighter than oxygen gas, the proportions, when 
measured by the bulk of the two gases, are eight 
measures of hydrogen to two measures of oxygen. 

Robert. — But how can it be known that water is a 
mixture of these two gases ? 

Mr. P. : — By burning a mixture of oxygen and hydrogen 
gases in a closed vessel, the product is found to be 
water ; and the quantity of water formed, is just equal 
to the weight of the gases burned. Again, water may 
be separated into its two gases by passing steam through 
a red-hot tube containing iron wire ; for in this case the 
oxygen in the steam, having a greater attraction for the 
hot iron than it has for the hydrogen, unites with the 
wire, and leaves the hydrogen, which passes along the 
tube, and may be collected separately. The oxygen 
will become solid in the iron ; and when the wire is 
taken out of the tube, it will be found to be covered 



FIRE. 97 

with rust, and to be heavier than before, in consequence 
of its combination with the oxygen that previously 
existed in the steam. 

Frederick. — Then, is rust solid oxygen, father ? 

Mr. P. — It is oxygen united with iron ; and the 
rusting of iron is owing to its great attraction for oxygen. 

Robert. — But if the burning of the fire be owing 
to the condensing of oxygen from the air into water, 
by mixing with hydrogen, where does the hydrogen 
come from for it to mix with 1 

Mr. P. — From the coals, which contain a great 
quantity of hydrogen in a solid state, in the same way 
as rusted iron contains oxygen. The hydrogen in coals 
may be driven from them in the state of gas, by merely 
beating them in closed vessels, to prevent the hydrogen 
from uniting with the oxygen of the air. It is in this 
manner that the gas is produced with which the streets 
and shops are so brilliantly lighted. 

Robert. — But it very often happens, that the fire does 
not blaze at all ; and it seems to be hotter when the 
coals are of a bright red heat than when they are blazing. 

Mr. P. — The red heat in the coals is produced by 
the union of the oxygen in the air with another substance 
in coals, that constitutes the principal part of their 
weight ; this substance is charcoal, called by chemists 
carbon. Oxygen has, indeed, a greater attraction for 
carbon than it has for hydrogen ; but their rapid union, 
so as to produce combustion, does not take place until 
exposed to a higher temperature than hydrogen. Now, 
when coals are heated in a fire-grate, the heat is sufficient 
to produce this rapid union between the carbon, or coke, 
in the coals, and the oxygen of the air ; and the red heat 
we see is occasioned by the oxygen uniting with the coke. 
The combustion that thus takes place is much slower 
9 



98 FIRE. 

than when oxygen unites with hydrogen, because the 
carbon, or coke, being solid, the union proceeds only 
on its surfaces ; whereas, in the union of the two gases, 
it takes place through the whole mass. When the 
combination of the oxygen and carbon is quickened by 
a blast of air from the bellows, the heat is greatly 
increased ; and in a blacksmith's forge it is so intense, 
that the light given out is more brilliant than that of 
common flame. 

Frederick. — Does the burning of charcoal and 
oxygen form water, as well as the burning of oxygen 
and hydrogen 1 

Mr. P. — No ; the result of their union is a transparent 
gas, containing the carbon in solution, and called carbonic 
acid gas. This gas is sometimes called " fixed air :" and 
it is the same as that which is compressed into soda water, 
and which rises in bubbles in bottled porter, and many 
other fermented liquors. When a piece of charcoal is 
burned in oxygen gas, it entirely disappears, and is dis- 
solved in the gas. The oxygen gas is then changed into 
carbonic acid gas, which is perfectly transparent, though 
it contains the charcoal in solution ; and the weight of this 
gas is exactly equal to the weight of the oxygen and char- 
coal together. 

Frederick. — Is the flame of a candle produced by the 
union of oxygen and hydrogen, or by oxygen and char- 
coal? 

Mr. P. — It is by both ; for tallow and oil are composed 
of carbon and hydrogen. When the wick of a candle is 
lighted, the heat produced is sufficient to enable the hy- 
drogen of the tallow to combine with the oxygen of the air, 
and form water. The carbon that was previously united 
with the portion of hydrogen consumed, is thus set at lib- 
erty ; and, if the heat be sufficiently great, it also unites with 



FIRE. 99 

a fresh portion of oxygen, and forms carbonic acid gas. 
But, as the carbon and oxygen require a greater degree of 
heat to enable them to combine than the hydrogen and ox- 
ygen, part of the carbon is frequently not burned, and rises 
in the form of smoke. 

Frederick. — Then the light from the flame of a candle 
is produced by two burnings of two separate quantities of 
oxygen ; first, by its uniting with the hydrogen in the tal- 
low, and then with the carbon. 

Mr. P. — Yes. The hydrogen, as it requires less heat 
to combine with the oxygen, is the first to inflame, and 
gives that blue light you may perceive on the first lighting 
of a candle ; then, as the carbon separates, and acquires 
sufficient heat, by the increase of the flame, to unite with 
the oxygen, it also burns. But, as the hydrogen is the first 
to take the oxygen from the air, it frequently happens that 
the supply of air to the middle of the flame is not sufficient 
to afford oxygen enough to burn all the carbon liberated 
from the tallow or oil. This is the cause of common lamps 
so frequently smoking, and the defect is remedied in the 
Argand lamps by allowing a current of air to pass through 
the middle of the flame. By this method a sufficient 
supply of oxygen is obtained, and the lamp burns with 
greater brilliancy in proportion to the quantity of air con- 
sumed. 

Harriet. — I should never have supposed that so many 
changes and mixtures were taking place as the candle 
flames away, and the coals are burning to ashes. 

Mr. P. — You must not suppose, Harriet, that the coals 
or the candle are absolutely consumed ; there is nothing 
destroyed by being burned ? 

Harriet. — Not destroyed, papa 1 Why, what becomes 
of the candle and the coals ? 

Robert. — I suppose they are hidden, Harriet; and that^ 



100 FIRE. 

if you only knew how to look for them, you would see can- 
dles and coals piled up latent in the air, as heat is. 

Mr. P. — Very good, Robert, and more true than you 
seem to imagine. When any combustible substance is 
burned, the elements of which it is composed are merely 
undergoing a change of form, and are no more destroyed 
than sugar is destroyed by being dissolved in a cup of tea. 

Robert. — Then what becomes of the tallow in a burn- 
ing candle, father ? 

Mr. P. — The hydrogen of the tallow, I have told you, 
forms water by its union with oxygen, while part of the 
carbon rises in smoke, and part of it also unites with oxv- 
gen, and forms a new kind of gas. When any substance 
is burned, the weight of the product, after burning, is 
greater, instead of being less, than it was before ; and the 
increase of weight is equal to the quantity of oxygen 
united with it. Thus, if we were to collect the product 
of a burned candle, we should find that the water, and the 
carbonic acid gas, and the smoke, would weigh more than 
the original weight of the candle ; and if it were burned in 
pure oxygen gas, we should find that the weight it had 
gained, was just equal to the weight of the oxygen gas 
consumed. Though we cannot reproduce the candle in 
the same shape as before, we can collect all the materials 
of which it was made, mixed with another ingredient (oxy- 
gen) that increases the weight of the product. 

Harriet.— Well, if fire cannot destroy things, I do not 
know what can. 

Mr. P. — Nor do I, Harriet. All substances are inde- 
structible ; and, during the apparent decomposition of 
bodies, their component elements are merely undergoing 
new changes, to re-appear in another form. The changes 
that are continually going on in the works of nature afford 
some of the most interesting subjects of contemplation that 



FIRE. 101 

can occupy the mind of man. In the vegetable world, 
for instance, we witness the progress of decay with each 
succeeding winter; and, to a spectator not favored with 
the light which science affords, the leaves rotting upon the 
ground may appear as symbols of rapid and total destruc- 
tion : but, to a more penetrating eye, the decomposition of 
those objects which, during the summer months, clothed 
the scene in beauty, is regarded as a necessary process for 
Tesolving the vegetable matter into its original elements, to 
be again employed in renewing the foliage on the return 
of spring. 



QUESTIONS. 

1 Of what is the air composed ? — What part of it is oxygen ? — 
nitrogen ? 

2. What causes combustibles to burn ? 

3. Will nitrogen gas support combustion ? 

4. How do bodies burn in oxygen ? — What is the product ? 

5. Of what is water composed ? 

6. Which is lighter, hydrogen gas or atmospheric air? — how 
much ? 

7. What are the proportions of hydrogen and oxygen in the com- 
position of water by weight ? — by bulk ? 

8. How can it be proved that water is a mixture of hydrogen and 
oxygen gases ? 

9. What is the rust of iron ? 

10. When a fire is burning, where does the oxygen come from ? — 
the hydrogen ? 

J 1 . What kind of gas is used for lights ? 

12. How is it produced ? 

13. How do you account for the heat of red coals when there is 
no blaze ? 

14. How is carbonic acid gas produced ? 

9* 



102 FIRK. 

15. In what liquors is this gas found r 

16. How is the flame of a candle produced ? 

17. Why does a lamp smoke, when the wick is too long ? 

18. What combinations take place in the burning of a candle 

19. Can any thing be annihilated by burning ? 

20. What becomes of the tallow in a burning candle ? 

21. Repeat the facts taught in this conversation. 



THE BAROMETER. 103 



CONVERSATION XII 

THE BAROMETER. 



Mr. P. — What a beautiful day this is! and I am 
glad to see the barometer promises a continuance of 
such weather. 

Harriet. — How is it, papa, that you can tell, by 
looking at the barometer, what kind of weather it is 
likely to be ? 

Mr. P. — The mercury in the tube rises or falls as 
the air becomes lighter or heavier, and we can generally 
tell by the weight of the atmosphere what kind of 
weather to expect. 

Frederick. — Then does the weight of the atmosphere 
vary, father ? I thought it was about equal to a pressure 
of fifteen pounds on every square inch. 

Mr. P. — That is about its usual pressure, but its 
actual weight is continually varying; as we know by 
the height of the column of mercury it will sustain in. 
the barometer. The extent of this variation in England 
is as much as three inches; that is, the atmosphere 
will at one time be capable of supporting a column of 
mercury thirty-one inches high, while at other times it 
will not support more than a column of twenty-eight 
inches. The weight of a column of mercury thirty 
inches high, and one inch square, is fifteen pounds; 
therefore each inch weighs half a pound, and it requires 
half a pound of pressure on every square inch to raise 
the mercury one inch in the tube. Thus you perceive 



104 THE BAROMETER. 

that the pressure of the atmosphere varies in England 
as much as a pound and a half on every square inch, 
which is one tenth part of its whole weight. 

Frederick. — Does the real weight of the air alter 
with its change of pressure on the earth ? 

Mr. P. — That is generally the case ; for the air being 
an elastic body, it expands, and, consequently, becomes 
lighter, when any of the pressure is removed.* Whenever 
part of the pressure from the air above is taken away, 
the air expands in proportion to the diminished pressure ; 
therefore, by knowing the change of pressure, you can 
generally ascertain its change of weight. Under certain 
circumstances, however, the pressure may continue the 
same, though the air becomes lighter, as I shall soon 
explain ; but, before we proceed farther, I will make 
you acquainted with the construction of the barometer. 

Harriet. — Shall you take yours to pieces, father? 

Mr. P. — No, Harriet ; I shall be able to show you 
the nature of it as well with this barometer tube. It is 
thirty-six inches long ; and when I fill it with mercury, 
and invert it in a cup full of the same fluid, part of the 
mercury will run out, leaving a vacuum at the top ; but 
a column, equal to the pressure of the atmosphere, will 
remain suspended in the tube. 

Harriet. — Why does it not all run out? 

Mr. P. — Because the pressure of the atmosphere on 
the outside prevents it. I will put the cup of mercury, 
with the tube, under the receiver of the air pump, and 
by exhausting the air the mercury will descend. (Mr. 
Powell adjusts the tube into the top of the receiver, and 
begins to exhaust the air.) Observe, the mercury falls 
as I remove the pressure. 

* The pupil will understand this remark as applying to the rarefied por- 
tion of air, and not to the whole mass of the atmosphere, as the question 
would seem to imply.— Am. Ed. 



THE BAROMETER. 



105 



Harriet. — Yes, I see it is now within a few inches 
of the quicksilver in the cup. 

Mr. P. — When I admit the air again, the pressure 
will force the mercury up the tube as high as before. 
Look ! it is rising rapidly. 

Harriet. — Yes, it is as high as ever. 

Mr. P. — It remains suspended about thirty inches 
from the surface of the mercury in the cup ; therefore 
the weight of a column of mercury of that height is 
equal to the weight of a column of air, of the same size, 
reaching to the top of the atmosphere. 

Frederick. — Then, I suppose, if a tube were filled 
with a liquid lighter than quicksilver, it would require 
a much higher column to balance the weight of the 
atmosphere. 

Mr. P. — You are right, Frederick. A column of 
water thirty-four feet in height would be required to 
balance the pressure ;* and of spirits it would require a 
still higher column. 



* Upon this principle depends the action of the sucking or common pump. 
(See Figure 4.) A B is the body of the pump ; which is a hollow cylinder, 
made of wood or metal. The bore of the pump 
Fig. IV. must be smooth and of a uniform size, that the piston, 

P, may move easily, and exactly fill it. The piston 
is moved up and down by means of a lever, D ; and 
it has a valve v, which opens upwards. A hollow 
plug, C, is fixed in the body of the pump, below 
the piston, which also is provided with a valve, x, 
that opens upwards. When the piston is raised, it 
carries up the column of air above it, and the air below 
the plug, C, immediately ascends through the valve x 
to occupy the place thus left vacant. As the piston 
descends, the valve x is closed by the pressure upon 
it, and the air above C is forced through the valve v, 
which closes as the piston begins to ascend again. 
In this way, a few strokes of the piston removes 
all the air from the pump ; and its place is supplied 



106 



THE BAROMETER. 



Frederick. — If the pressure of the atmosphere depend 
upon the height and weight of the column of air above 
us, I should suppose that at the top of high hills the 
pressure must be less, as the column of air above must 
there be shorter. 

Mr. P. — That is exactly the case ; and the mercury in 
the barometer falls when the instrument is elevated. 
Indeed, a very sensitive barometer would indicate a 
difference in the weight of the air on the ground and on 
the chimney-piece. The barometer is now commonly 
used to measure the heights of mountains. 

Robert. — How much does the quicksilver fall as the 
barometer ascends? 

Mr. P. — It is ascertained that the weight of a column 
of air eighty-seven feet high, (when the atmosphere 
supports thirty inches of mercury,) is equal to the weight 
of a column of mercury one tenth of an inch high ; 
therefore, when the barometer is raised eighty-seven feet 
in the air, the mercury falls one tenth of an inch. 

Robert. — Would it fall one tenth of an inch for every 
eighty-seven feet of height ? 



by water, which is forced up by the atmospheric pressure upon the surface 
of the water in the well. 



Fig. V. 




The action of the forcing pump (Fig. 5.) 
is very much like that of the sucking pump ; 
but its piston is not furnished with a valve, 
jggs When it is forced down, therefore, as the 
^ water cannot descend through the valve x> 
which is closed, it is driven out with violence 
through the valve v in the spout, which opens 
outwards. 

The fire-engine is constructed on the principle 
of the forcing pump. The water is forced, 
by two alternating pistons, into an air-vessel; 
from which it is driven, in a constant stream, 
by the elastic power of the condensed air. — 
Am. Ed. 



THE BAROMETER. 107 

Mr. P. — No, my dear ; for as we ascend higher, the 
air becomes more rarefied, in consequence of the pressure 
upon it being diminished ; therefore, a second column of 
eighty-seven feet will not weigh so much as the first, and 
the barometer must be elevated more than eighty-seven 
feet, to cause the mercury to fall one tenth of an inch. 
At the height of seven hundred feet, for instance, the 
weight of a column of air eighty-nine feet high is only 
equal to the weight of a column of eighty-seven feet 
near the ground. 

Harriet. — Why is a thermometer fixed to a barometer 
frame 1 

Mr. P. — The weight of the mercury in the barometer 
tube varies according to the change of temperature, as 
I showed you in the experiment with heated mercury in 
a former conversation. When the mercury is made 
lighter by heat, it rises higher in the tube, even when 
no change has taken place in the pressure of the air. 
In accurate observations, therefore, it is necessary to 
know the degree of expansion, that allowance may be 
made for it, in calculating the weight and pressure of 
the atmosphere. 

Harriet. — What makes the air heavier at one time 
than at another 1 

Mr. P. — That is a very puzzling question, Harriet, 
and one on which philosophers are not agreed. 

Robert. — I should think there cannot be much 
difficulty in the matter ; for as heat expands air, and 
makes it lighter, the change must depend upon hot and 
cold weather. 

Mr. P. — There is much more difficulty, Robert, than 
you conceive. Most persons, who have not attended to 
the subject, would, I dare say, think with you, that, when 
the air is lightest, the pressure is the least ; but we find 



108 THE BAROMETER. 

that, in the hottest weather, the barometer is often 
the highest. 

Robert. — But if the pressure of the air be owing 
to its weight, the pressure must be least when the air 
is lightest. 

Harriet. — Yes, papa, I think Robert must be right 
there. 

Mr. P. — I do not wonder at your thinking so; and 
those persons, who are more pleased with sounds than 
with philosophical researches, would pronounce Robert's 
logic to be unanswerable, and, without paying more 
attention to the matter, be content to remain in ignorance 
all their lives. 

Frederick. — I confess I thought what Robert said 
was very true ; for if a Column of air reaching from the 
ground to the top of the atmosphere weighs, when the 
air is heavy, only fifteen pounds, how can it weigh as 
much when the air is light 1 

Mr. P. — If we take for granted, as you seem to do, 
that the whole atmosphere is equally affected by change 
of temperature, then the expansion of the air by heat 
would raise the atmosphere higher; and in this way 
it might make up, by its increased height, for the loss 
of specific gravity. 

Frederick. — I had forgotten that the atmosphere 
might rise higher at one time than another. 

Robert. — But is the atmosphere raised higher in hot 
weather than in cold ? 

Mr. P. — We may, perhaps, safely conclude that in 
summer time, when the air in this part of the globe is 
more heated than in winter, our atmosphere is actually 
higher; but partial changes of temperature near the 
surface of the earth, of mere local extent, can have but 
a slight effect in elevating the atmosphere. 



THE BAROIUETER. 109 

Robert. — But if the pressure of the atmosphere be 
not changed by the air being made lighter, I cannot think 
what else can produce any change in its weight. 

Mr. P. — The principal cause of the variation of the 
pressure I believe to be the wind. 

Robert. — How can the wind alter the pressure of 
the atmosphere ? 

Mr. P. — The horizontal motion of the air may 
diminish its pressure. Air is subject to the same laws 
that regulate other liquids in motion, and we find that 
a quick horizontal motion communicated to fluids, not 
only diminishes their perpendicular pressure, but absolutely 
suspends it altogether. Thus we see water spouting out 
horizontally to a considerable distance from a hole at the 
bottom of a water-tub ; the gravitation, or weight of the 
water, being suspended by the force with which it is 
impelled in a horizontal direction. In the same man- 
ner we may suppose the weight of the atmosphere be- 
comes affected by currents of air, which, when in motion, 
have the whole or part of their weight suspended : and 
the pressure is lessened according to the depth of the 
current and the rapidity of its motion. 

Robert. — The effect of motion on the weight of the 
air is, then, the same as upon solid bodies ; for I know that 
ice will often bear a person, when skating quickly over it, 
that would break if he were to stand still. 

Mr. P. — Yes, that is a good illustration of the effect 
of horizontal motion in diminishing perpendicular pressure. 
The throwing of stones, the firing of cannon balls, &c, 
are also instances of the same power exerted on solid 
bodies, and suspending, for a time, all their weight. 
I can show you a very curious experiment, that will 
illustrate the effect of horizontal motion upon air most 
remarkably. 

10 



UO THE BAROMETER. 

Harriet. — Pray do, papa. 

Mr. P. — You perceive that in the side of this tube, 
open at both ends, there is a small hole. Now, I will 
place this hole close to the flame of the candle while I 
blow through the tube ; and you will observe that the 
flame, instead of being blown away, as you might suppose, 
v/ill be drawn in towards the hole. (Mr-. Powell bloics 
through the tube, and the flame is evidently attracted to the 
hole, at which the children express much surprise.) 

Robert. — Well, I cannot think how that can be, if 
you blew, father, and did not draw in your breath. 

Mr. P. — You may try the experiment yourself, Robert. 
(Robert bloics through the tube, 
Fig. VI. in the manner represented in the 

annexed wood-cut, with the same 
residt ; and then Frederick and 
Harriet also try the experiment 
with similar success.) You seem 
to be quite astonished ; but the 
wonder is easily explained on 
the principle of horizontal motion removing atmospherical 
pressure. While the air inside the tube was at rest, it 
exerted a pressure against the interior, equal to the pres- 
sure of the atmosphere on the outside ; consequently, both 
forces being equal, they balanced each other. But when 
you blow through the tube, the pressure of the air against 
the inside is diminished by the motion, and the pressure 
of the atmosphere on the outside forces the air into the 
hole, and carries with it the flame of the candle. 

Robert. — Then the harder you blow, the greater will 
be the draught of air into the hole 1 

Mr. P. — Yes, if the tube be enlarged towards the end, 
to admit a more free passage of the air ; but if the air 




THE BAROMETER. Ill 

be obstructed in its progress, it will be compressed, and 
force itself through the hole. 

Harriet. — It is a very curious experiment. 

Mr. P. — The same effect may be produced by blowing 
between two slips of paper, about six inches long, and 
three inches wide. If you place the slips of paper over 
one another, and hold two of the corners together, so that, 
when applied to the mouth, you can blow between them at 
their narrowest edge, the effect of the blast of air will be to 
bring the two slips of paper closer than before, instead of 
to separate them farther, as you might suppose it would. 
These experiments afford a very good illustration of the 
effect of wind in lessening the pressure of the atmosphere. 

Robert. — Does the barometer fall during wind ! 

Mr. P. — Yes ; the mercury never falls so low, nor so 
suddenly, as during a high wind. 

Robert. — But if the motion of the air take off the 
pressure, it ought, during a very high wind, to remove 
it altogether. 

Mr. P. — And so, perhaps, it would if the whole atmos- 
phere were in rapid motion. The winds near the earth, 
however, do not extend very high in the atmosphere, and 
a much greater proportion of the air has its perpendicular 
pressure undisturbed. 

Frederick. — But does not the barometer often fall 
when there is no wind ? 

Mr. P. — It often varies when the wind is scarcely 
perceptible to us ; but there are continual currents in 
the upper air, which we cannot perceive ; and it is to 
these that the changes in the pressure are to be generally 
attributed. 

Frederick. — Does the barometer vary as much in all 
parts of the world as it does in England 1, 

Mr. P. — Not by any means. Its greatest variations 



112 THE BAROMETER. 

take place in the temperate zone. Within the tropics, 
indeed, where the wind generally blows in one direction 
for months together, the barometer seldom varies, except- 
ing during storms. This fact confirms the opinion that 
these changes are owing to the winds. In our own vari- 
able climate, the wind is not often settled for three days 
following : but when it does continue to blow from one 
quarter, we generally find the barometer either to rise or 
fall during the whole time. 

The changes, that are continually taking place in 
the pressure of the atmosphere, are severely felt by per- 
sons in delicate health ; and it is only surprising that we 
do not feel them more. To raise the mercury one inch, 
the pressure of the atmosphere must have increased 
half a pound on every square inch of surface of our 
bodies ; it must, therefore, increase the whole external 
pressure, on an average, about eight hundred pounds. 
A change to this extent often occurs in a few hours ; 
yet most of us are not aware of any change in the 
pressure on our bodies. The whole pressure of the atmos- 
phere, on the body of a moderate sized man, may be es- 
timated at twenty-five thousand pounds. 

Frederick. — Can you explain to us, father, how it is that 
we are able to bear so great a pressure without feeling it 1 

Mr. P. — The elasticity of the blood, and of the air con- 
tained in the body, counteracts the effects of the external 
pressure. When the weight of the atmosphere is in- 
creased, the change is generally communicated, through the 
lungs, to all the animal fluids so gradually, that the va- 
riation is not perceived ; but when the change is so sudden 
as not to allow time for the densities of the fluids and air 
vessels to accord with the altered pressure, considerable 
pain is experienced. In diving-bells, for instance, as the 
weight of water above compresses the air within, if the bell 



THE BAROMETER. 113 

vered quickly, a most painful sensation is produced. 
particularly in the eves and ears, owing to the increased 
pressure. 

Thomas. — You have not told us vet, father, how it \s 
that the barometer can tell what kind of weather we are 
to have. 

Mr. P. — Not long after the invention of the barometer, 
3 discovered, that when the mercury was high, the 
weather was generally tine ; and, on the contrary, that the 
fall of the mercury was followed by rain. It was. therefore, 
called a weather glass. The cause of these phenomena, 
attending the rise and fall of the barometer, is not yet very 
accurately ascertained.* The commonly received opinion 
is. that, as the air becomes light and unable to sustain the 
clouds, they descend in rain. 

Frederick. — Does rain always follow after the fall of 
the mercury ? 

* The following rules are given in Lardner's Pneumatics .— 

1. Generally, the rising of the mercury indicates the approach of fair 
weather; the falling of it shows the approach of foul weather. 

2. In sultry weather, the fall of the mercury indicates coming- thunder. 
In winter, the rise of the mercury indicate s frost In frost, its fall indicates 
a thaw ; and its rise indicates snow. 

3. Whatever change of weather suddenly follows a change of the ba- 
rometer, may be expected to last but a short time. Thus, if fair weather 
immediately follow the rise of the mercury, there will be but litde of it : and 
in the same way. if foul weather follow the fall of the mercury, it will last 
but a short time. 

4. If fair weather continue for several days, during which the mercury 
continually falls, a long succession of foul weather will probably ensue ; 
and again, if foul weather continue for several days, while the mercury 
continually rises, a long succession of fair weather will probably succeed. 

5 A fluctuating and unsetded state in the mercurial column, indicates 
changeable weather. 

to be observed, that the changes of weather are not indicated so 
much by the height of the mercury, as by its change of height. Of course. 
the words "Fair." u Wind? " R.:in." 6cc commonly engraved on the 
plates of barometers, are of little or no use. — Am. Ed. 

10* 



114 THE BAROMETER. 

.Mr. P. — Very frequently not ; and sometimes, indeed, 
we have fine weather when the barometer is very low ; and, 
on the contrary, we have rain when the mercury is high 
in the glass. The barometer will often be rising gradually, 
day after day, though it is raining all the time ; but when 
the barometer has risen or fallen for several successive 
days, such rise or fall is generally followed by fine or 
stormy weather. We can, in general, judge with more 
correctness of the weather by knowing whether the mer- 
cury be rising or falling, at the time, than by the height at 
which it actually stands. Thus, if the mercury be at 30 
inches, which is marked " Fair" on the scale, yet if we find, 
on gently shaking the barometer, (to disengage the mer- 
cury from the glass,) that it is falling lower, we may ex- 
pect rain. 

Our conversation has been longer than usual this morn- 
ing ; but the pressure of the atmosphere varies so materially 
in this country, and the causes of its variation are, gen- 
erally, so little understood, that I thought it advisable to 
enter fully into the subject. It is one attended with con- 
siderable difficulty ; and though, to me, the explanation 
I have given of the changes in atmospheric pressure, 
seems sufficient to account for all the phenomena, there 
are many persons who think it necessary to adduce various 
other causes to explain them. The vapor in the atmos- 
phere is supposed by some to be chiefly instrumental in 
producing the changes in the barometer ; but it can be 
readily shown, that the mercury rises and falls quite inde- 
pendently of the quantity of vapor in the air. Upward and 
downward currents of wind are also supposed to have 
great influence on the weight of the atmosphere ; but the 
horizontal motion of the air appears to me to be sufficient 
to account for all the changes in its pressure. 



THE BAROMETER. 115 



QUESTIONS. 

1 . la the weight of the atmosphere uniformly the same ? 

2. By what instrument is the weight of the atmosphere ascer- 
tained ? 

3. What is the ordinary height of the mercury in the barometer ? 

4. What pressure is required to raise the mercury one inch ? 

5. Does the real weight of the atmosphere alter ? — Why ? 
C. Describe the barometer. 

7. How is the column of mercury sustained in the tube ? 
S. What experiment proves that it is sustained by atmospheric 
pressure ? 

9. To what is the weight of the column of mercury equal ? 

10. How high a column of water will the atmosphere support ? 

11. By what is the water raised in a common pump ? 

12. In what manner ? — Describe Figure 4. 

13. How is water raised by a forcing pump ? — Explain Figure 5. 

14. Is the pressure of the atmosphere the same on a hill and in a 
valley ? — Why not ? 

15. How much does the quicksilver fall as the barometer ascends ? 

16. Does it fall uniformly at different heights ? 

17. Why is a thermometer fixed to a barometer frame ? 

18. Can you tell how the heights of mountains are ascertained 
by means of the barometer ? 

1 9. What makes the air heavier at one time than at another ? 

20. HoW can the wind alter the pressure of the atmosphere ? 

21 . By what examples is this theory illustrated ? 

22. Describe the experiment represented by Figure 6. ' 

23. How do you account for the result of this experiment ? 

24. What effect has wind upon the barometer ? 

25. Why is not the pressure of the atmosphere wholly removed 
by a very high wind ? 

26. What makes the barometer fall when there is no wind ? 

27. Where does the barometer vary most ? 

28. What is said of the tropical regions ? 

29. How great is the pressure of the atmosphere upon a man ? 

30. How are we enabled to bear so great a weight ? 

31. In what manner are changes of the weather indicated by a 
barometer ? 



1 16 WINDS. 



CONVERSATION XIII 



WINDS. 



Mr. P. — As I yesterday attributed all changes in the 
pressure of the atmosphere to the motion of the air, we 
will now proceed to consider the causes of that motion. 
Can any of you form an idea upon the subject? 

Frederick. — The winds may be caused, perhaps, by 
the heat of the sun expanding the air. 

Mr. P. — You are very near the truth, Frederick. 
When a portion of the atmosphere near the earth is ex- 
panded by heat, it becomes lighter than the air above, and 
ascends till it arrives at a stratum of air as light as itself. 
The surrounding heavier air rushes to supply the place of 
that which is ascending ; this air, in its turn, also becomes 
heated, and ascends ; and in this manner a current of air 
is produced, rushing towards the heated space. 

Frederick. — Does not the same thing happen in a 
room with a fire ? for we always find a draught of air 
from the door and windows towards the fireplace. 

Mr. P. — That is a very correct illustration, Frederick. 
The air, as it. is heated, rises up the chimney, and the 
colder and heavier air rushes into the room to supply its 
place. Were the earth and sun stationary, the wind 
would be always blowing towards the same point ; but the 
diurnal revolution of the earth, and its motion round the 



WINDS. 117 

sun, cause a constant change in the part most heated by 
the sun's rays. 

Robert. — But as the sun rises in the east and sets in 
the west, the wind ought always to follow in the same 
direction. 

INLi. P. — And so, in all probability, it would, if our 
Tiobe were covered with water, instead of being divided 
into water and land, mountains and valleys. 

Robert. — How can the hills and valleys and seas, 
alter the winds ? 

Mr. P. — In a variety of ways. The sun's rays, when 
striking upon the ocean, penetrate deep into the body of 
the water, heating the whole mass ; therefore, a compar- 
atively small portion of heat is reflected into the air. 
The greater evaporation from water, too, cools its surface ; 
and both causes contribute to prevent the air, over water, 
from being nearly so much heated, as the air on the surface 
of dry land. The hills and mountains act as so many 
screens to check and alter the course of the wind. They 
also cool the air as it passes over their snow-capped 
summits; and in consequence of the elevation of the 
land, the temperature of many places, even under a 
vertical sun, is much colder than any we experience in 
this country. Every circumstance, that tends to change 
the temperature of the air, produces a variation in the 
winds : they must even be affected by the passing clouds, 
which screen the earth from the direct heat of the sun, 
and thereby check the expansion of the air. 

Frederick. — Do the winds, then, never blow regularly ? 

Mr. P. — Yes ; in those parts of the world where the 
causes I have mentioned, as producing changes in the 
wind, do not operate with sufficient force to counteract the 
power of the sun. On the Atlantic and Pacific Oceans, 
the wind blows constantly in the direction of the sun's 



118 WINDS. 

course, throughout nearly all the space included between 
the 30th degrees of north and south latitude. These 
winds are called the trade-winds, from their great impor- 
tance to navigators : their direction is nearly east at their 
farthest limits from the equator, but they gradually 
incline towards the equator as they approach it.* 

* If our globe were at rest, and the sun were always acting- over the 
same part, the earth and air directly under him would become exceedingly 
heated, and there, the air would be constantly rising like the smoke from a 
great fire ; while currents or winds would be pouring towards the central spot 
from all directions below. But the earth is constantly turning around under 
the sun, so that the whole middle region or equatorial belt may be called 
the sun's place} and, therefore, there should be over it a constant rising of 
air, and constant currents from the two sides of it, or the north and south, 
to supply the ascent. Now this phenomenon is really going on, and has 
been going on ever since the beginning of the world, producing the steady 
winds of the northern and southern hemispheres, called trade-uinds. 

The trade-winds, however, do not appear on the earth to be directly north 
and south, as they are in fact ; for the eastward whirling or diurnal rotation 
of the earth, causes a wind from the north to appear as if coming from the 
north-east, and a wind from the south as if coming from the south-east. 
This is illustrated b}^ the case of a man on a galloping horse, to whom a 
calm appears to be a strong wind in his face; or if he be riding eastward, 
while the wind is directl} 7 north or south, such wind will appear to him to 
come from the north-east or south-east. Thus, it is the whirling of the earth 
which is the cause of the oblique and westward direction of the trade-winds, 
and not, as has often been said, the sun drawing them after him. 

The reason why the trade-winds, at their external confines, appear almost 
directly cast, and become more nearby north and south as they approach the 
central line, is, that at the confine they are like fluid coming from the axis 
of a turning wheel, which has approached the circumference, but has not 
yet acquired the velocity of the circumference ; while nearer the line, they 
are like the fluid after it has, for a considerable time, been turning on the 
circumference, and has acquired its rotatory motion ; appearing at rest as 
regards that motion, but still leaving sensible any motion in a cross 
direction. 

While, in the lower regions of the atmosphere, air is constantly flowing 
towards the equator and forming the steady trade-winds between the tropics, 
in the upper regions there must, of course, be a counter-current distributing 
the heated air again over the globe. Accordingly, since reasoning led men 
to expect this, many striking proofs have been noted. At the summit oft' 1 - 



WINDS. 119 

Frederick. — Would not the trade-winds blow as 
regularly, if that part of the world were dry land instead 
of being covered with water 1 

Peak of Teneriffe, observations now prove that there is always a strong- 
wind blowing in a direction contrary to that of the trade-wind on the face of 
the ocean below. Again, the trade-winds among the West-India Islands 
are constant ; vet, volcanic dust, thrown aloft from the Island of St. Vincent, 
in the year 1812, was found, to the astonishment of the inhabitants of Barba- 
does, hovering over them in thick clouds, and falling, after coming more 
than a hundred miles directly against the strong trade-wind, which ships 
must take a circuitous course to avoid. In sailing from the Cape of Good 
Hope to St. Helena, the sun is often hidden, for days together, by a stratum 
of dense clouds passing southward high in the atmosphere; which clouds 
consist of the moisture raised near the equator with the heated air, 
and becoming condensed again as it approaches the colder regions of 
the south. 

Beyond the tropics, where the heating influence of the sun is less, the 
winds occasionally obey other causes than those we have now been con- 
sidering, which causes have not yet been fully investigated. The winds of 
temperate climates are, in consequence, much less regular, and are called 
variable ; but still, as a general rule, whenever air is moving towards the 
equator, from the north or south poles, where it was at rest, it must have 
the appearance of an east wind, or a wind moving in the contrary direction 
to the earth itself, until it has gradually acquired the whirling motion of that 
part of the surface of the earth on which it is found ; and again, when air 
is moving from the equator, where it had at last acquired nearly the same 
motion as that part of the earth, on reaching parts nearer the poles, and which 
have less eastward motion, it continues to run faster than they, and becomes 
a westerly wind. In many situations beyond the tropics, the westerly 
winds, which are merely the upper equatorial currents of air falling down, are 
almost as regular as the easterly winds within the tropics, and might also 
be called trade-winds .-—witness the usual shortness of the voyages from 
New York to Liverpool, and the length of those made in the contrary 
direction. North of the equator, then, on the earth, true north winds appear 
north-east, and true south winds appear south-west. 

The theory of the winds, given in this note, is abridged from Arnott's 
Elements of Physics. The counter-currents of air, flowing towards and 
from the equatorial regions, may be illustrated in a heated room. If a 
lighted candle be held at the bottom of the door, the flame will be blown 
inwards ; but if it be held at the top of the door, it will incline outwards ; thus 
showing that the currents of air, at the top and bottom of the room, flow in 
different directions.— Am. Ed. 



120 WINDS. 

Mr. P. — No; their regularity is owing to the absence 
on the ocean of the causes which produce changes in the 
wind on land ; the water of the ocean being there of 
nearly one equal temperature, and presenting no elevations 
to check or alter the course of the wind. Even the trade- 
winds become changeable near land, and in the Indian 
Ocean their regular course is so much affected by the 
changeable temperature of the land on the continent of 
Asia, that, instead of blowing throughout the year in one 
direction, the winds blow for six months from the north- 
east, and for six months from the south-west. This 
change takes place as the position of the sun is removed 
from the northern to the southern side of the equator : 
these winds are called monsoons. 

Robert. — How can the land on the continent of Asia 
produce this regular change ? 

Mr. P. — During our summer months, when the sun 
is north of the equator, the land on the north of the 
Indian Ocean becomes intensely hot, and the air, being 
rarefied, rises, and causes a current of wind to set in 
from the sea towards the land, to supply the place of the 
ascending air. When the sun is south of the equator, the 
land becomes cooler than the Indian Ocean, and the 
current of air is consequently changed. 

The different effects of water and land in expanding 
the air, and varying the course of the wind, are strikingly 
exhibited in the land breezes and sea breezes of tropical 
climates. The sea breeze sets in every morning about 
ten o'clock, and blows with delightful freshness towards 
the shore ; and in the evening the land breeze begins to 
blow towards the sea, and continues in that direction 
throughout the night. The sea breeze is produced by the 
greater rarefaction of the air over land during the day, 
which causes the cooler air from the sea to rush towards 



WINDS. 121 

the shore. The land breeze, on the contrary, is owing to 
the more rapid cooling of the land during the night ; for 
the air, as it cools, being condensed, a partial vacuum is 
formed, which is supplied from the upper atmosphere ; 
^nd the land air, being then heavier than the air over the 
water, rushes towards the ocean as the lighter air ascends. 

Harriet. — I have been thinking, father, that, as the 
world is always turning round, its motion must move the 
air, and cause a wind. 

Mr. P. — The air moves round with the earth; there- 
fore, as we are all moving together, we are not sensible of 
its motion. If the air did not move with the earth, we 
should be forced against it with so much velocity as to 
produce a wind ten times stronger than that of a hurri- 
cane ; nothing could withstand the force of its resistance ; 
and all objects, exposed to the action of such a wind, 
would be either levelled with the ground or carried 
round the world, 

Frederick. — Does not the motion of the earth, then, 
give any motion to the air, that we can perceive ? 

Mr. P. — It is supposed by many philosophers, that the 
rotary motion of the earth causes the trade-winds to blow 
from the east instead of from the north, in consequence of 
the earth at the equator moving much more rapidly than it 
does farther north or south. 

Frederick. — How is that, father ? 

Mr. P. — If you look at this globe, you will perceive 
that the different circles drawn round it, to mark the 
latitudes, diminish in size as they approach the poles. 
Now, these circles represent the comparative spaces 
through which any point upon them moves at one revo- 
lution. This circle, near which St. Petersburgh is placed, 
is only one half the size of that round the middle of the 
globe ; yet, as both move round in the same time, any 
11 



122 WINDS. 

point on the central circle must move twice as fast to 
complete its journey as St. Petersburgh, which is in the 
sixtieth degree of latitude. 

Frederick. — Yes, I see, as I turn the globe round, that 
the parts near the equator move so quickly, that I cannot 
distinguish them; while the places rear the pole are 
moving so slowly, that I can read their names. How is 
this difference in the motion supposed to alter the direction 
of the trade-winds? 

Mr. P. — Those, who ascribe the effect to this cause, 
take it for granted that the wind from northern latitudes, 
in moving towards the tropics, must have a motion 
directly south. Now, as the air at the equator is moving 
round with the world at the rate of about 1000 miles an 
hour, while the air at St Petersburgh is moving with only 
half that velocity, if a portion of the air from St. Peters- 
burgh could be conveyed in one hour to the equator, when 
it arrived there it would be 500 miles west of the longitude 
of that capital, because the land at the equator would 
have moved through a space of 1000 miles in the hour, 
while the rotary motion of the St. Petersburgh air would 
be only 500 miles in the same time. If the same air 
were to be twenty-four hours in completing its journey to 
the equator (still retaining its comparatively slow rotary 
motion of 500 miles an hour, while the equatorial land 
was moving at the rate of 1000), by the time of its arrival 
there, the air at the equator would have completed one 
whole revolution, while the air from the north would have 
completed but one half, and would arrive at the equator on 
the opposite side of the globe to that from whence it' 
commenced its journey. All objects turning round with 
the globe, on coming in contact with air not moving so 
fast as themselves, would strike against the slow-moving 
air, and the sensation produced would be the same as if 



WINDS. 123 

the wind were blowing against them. Thus, such a wind 
as we have been supposing, though really not travelling so 
fast as other objects by 500 miles an hour, would appear 
to be blowing against them with that degree of velocity ; 
and as the motion of the earth is from west to east, the 
wind would seem to be blowing from east to west. 

Robert. — Just in the same way, I suppose, as the 
wind always seems to me to be blowing in my face when I 
am running. 

Mr. P. — Yes ; just so. 

Robert. — But if the world really does slip from under 
the air, winds blowing south must lose ground, and appear 
to be blowing east ; and that would really account for the 
east trade-winds. 

Mr. P. — Yes, Robert, if the world do slip from under 
the air ; but there appears to be no sufficient reason to 
imagine that it does so to an appreciable extent. When 
any portion of the air in the northern hemisphere is 
moving south, towards the tropic of Cancer, this motion, 
we may presume, is caused by the rarefaction of the air 
drawing or impelling the wind toward some particular 
point ; and as this point revolves with the earth, we 
must suppose that the attraction would be sufficiently 
powerful to draw towards that point the air which it has 
once set in motion, so long as the attractive power 
continued in force. For instance, suppose a rarefaction 
of the air over the great sandy desert of Africa were to 
attract the air over London, and cause it to move directly 
southward at the rate of twenty-five miles an hour, the 
force of this attraction would prevent the air from shifting 
westward, and would continue to draw it toward the same 
point so long as the cause was in operation, however much 
faster that point might be moving round than the air it 
attracts. Wind, travelling from London to the desert of 



124 WINDS. 

Africa, at the rate of twenty-five miles an hour, would 
have an average tendency westward of about four miles 
at the end of each hour ; but the attraction southward, 
and the friction from the surface of the earth and 
from the air, in the course of twenty-five miles, would be 
sufficient to communicate to the air the extra rotary 
motion of the land near the equator as the wind proceeded 
southward.* 

Frederick. — Is wind produced by any other cause 
besides heat? 

Mr. P. — As the attractions of the sun and moon are 
sufficiently powerful to draw the water of the ocean from 
its level, and to cause the tides, we may presume that 
they would have great power in attracting the more 
volatile air. If this be the case, the atmosphere must be 
subject to tides as well as the ocean. Though these ebbs 
and flows of the air may not be perceptible near the earth, 
any more than the ebbs and flows of the ocean are at the 
bottom of the sea, yet they may exert considerable 
influence in altering, checking, and accelerating the 
upper currents of air.f 

* The oblique motion of water poured upon a globe revolving- round a 
perpendicular'axis has been adduced as an illustration of the manner in 
which the air in moving southwards receives a westerly direction; but tnis 
illustration is far from being a correct representation of the motion of 
the air; for the point of attraction, drawing the water to the ground, is 
stationary ; whereas, in the case of the winds, the attracting power revolves 
with the globe. — It has been considered advisable to give the foregoing 
aceount of Hadleys rotary theory of the trade-winds, and to state some of 
the objections that may be urged against it, though the subject docs not 
altogether come within the province of the present work, because this theory 
has been stated in some elementary works in deservedfy great repute, as if 
it were indisputably true ; and the juvenile mind might thus be led to 
consider a very questionable, if not altogether erroneous, theory, as a 
fundamental truth. 

f Among the many theories advanced to account for the phenomena of 
the winds, one of the most novel is that lately published in an American 



WINDS. 125 

Robert. — You did not mention the moon, father, yes- 
terday, in speaking of the pressure of the atmosphere ; 
yet it must have great power in altering the pressure, if it 
can draw the air after it. 

Mr. P. — No, I did not, because I conceive the chief 
influence the moon can possess, in varying the pressure 
of the atmosphere, must depend upon its producing upper 
currents of air ; and the cause of these currents I deferred 
speaking of until we considered the causes of the winds. 

Robert. — But if the moon raise the atmosphere higher 
without lessening the weight of the air, surely the pressure 
must be greater in that part 1 

Mr. P. — No, Robert ; the attraction of the moon would 
be fully adequate to keep off the pressure of the extra 
quantity of air it accumulated ; therefore, though the 
quantity of air would be heavier if the moon's attraction 
were removed, yet as that attraction would continue as 
long as the atmosphere was raised by it, the pressure 
would not be increased. 

Frederick. — How fast does the wind usually travel? 

Mr. P. — In a very high wind, the air travels at the 
rate of one hundred miles an hour. Aeronauts have 
been carried in balloons the distance of seventy miles 
within the hour ; and yet they could not have travelled as 
fast as the wind itself. 

Frederick. — The power of the wind must be very 
great when blowing at such a rapid rate. I wonder their 
balloon was not blown to pieces. 

Mr. P. — As the machine sailed along with the current, 

scientific journal, in which all air in motion is supposed to be revolving* 
round a central point. This revolving; motion would account for the fre- 
quently observed partial actions of storms, but it would scarcely be recon- 
cilable with the general motion of the winds j nor does the proposer of the 
theory show very clearly how the air could receive this gyratory motion* 
11 



126 WINDS. 

there would be little resistance, and the aeronauts would 
scarcely feel any wind. Had the balloon been fixed, it 
would have been blown to pieces instantly. The force 
of the wind increases according to the squares of the 
velocity, so that when the velocity of the wind is doubled, 
its force is increased four times. 

Harriet. — What is the force of wind when it is 
blowing one hundred miles in an hour ? 

Mr. P. — Such a wind, it is calculated, would act with 
a power of forty-nine pounds on every square foot of 
surface presented directly to its action. 

Harriet. — No wonder, then, that chimneys are blown 
down and trees uprooted by a high wind, if it have so 
much force. I am only surprised that more damage is 
not done. 

Mr. P. — The wind does not often travel at so rapid 
a rate as one hundred miles an hour. It is estimated that 
what is termed a " high wind" travels at the rate of about 
thirty miles an hour, at which rate it would strike objects 
with a force of upwards of four pounds on every square 
foot. Those objects only whose surfaces are placed 
directly against the wind feel its full effect. If the surface 
be rounded, or placed obliquely to the wind, a great part 
of the force is lost, and the air is reflected obliquely from 
the object. In the same manner, when you throw a ball 
obliquely against a wall, it bounds off in a direction 
equally oblique, and does not strike the wall with nearly the 
force it would have done if thrown directly against it. — The 
subject of reflection is one well deserving of particular 
investigation, as it will serve to explain many phenomena 
of common occurrence ; I shall, therefore, reserve it for 
a future occasion. 



WINDS. 127 



QUESTIONS. 

3 . How are the winds caused ? 

2. Give an illustration. 

3. Why does not the wind, at the equator, always blow from east 
to west ? 

4. How can mountains, valleys, and seas, alter the wind ? 

5. Where do the winds blow regularly ? 

G. What are the trade-winds ? — Why so called ? 

7. What are the monsoons ? — Where ? — How caused ? 

8. Give an account of land and sea breezes. 

9. How are they produced ? 

10. Can you give a reason for the eastern winds, which prevail 
on our sea-coast during spring and summer ? 

11. Why does not the motion of the earth cause a wind ? 

12. If the air did not partake of the earth's motion, what would 
be the consequence ? 

13. Does the motion of the earth give any motion to the air ? 

14. Explain the theory on this subject. 

15. Is wind produced by any other cause besides heat ? 

16. What is said of tides in the air? 

17. Does the barometer indicate the existence of such tides ? 

18. Why is not their pressure felt ? 

19. What is the velocity of the wind ? 

20. By what law does the force of the wind increase ? 

21. What is the force of a wind, which moves a hundred miles 
an hour ? 

22. According to what law is the force of the wind increased ? 

23. What is the velocity of what is called a " high wind ?" 

24. What is the force of such a wind? 

25. Give a general view of the doctrines explained in this 
conversation. 



128 LIGHT. 



CONVERSATION XIV 



LIGHT. 



Mr. P. — As I intend to explain to you, in the course of 
our present conversations, the nature of vision, it is advi- 
sable you should previously have some acquaintance with 
the properties of light, by means of which the organs of 
vision are brought into action. We cannot do better, 
therefore, than direct our attention to the subject this 
morning. 

Robert. — Why, what is there to be said about light, 
father, that we do not all of us know already 1 

Mr. P. — There is a great deal more to be learned about 
it, Robert, than any one at present knows ; and more, per- 
haps, than will be ever discovered by the mind of man. 
Even respecting the very nature of light, philosophers are 
at fault ; but it is generally supposed to be a material sub- 
stance, composed of particles infinitely small. 

Harriet. — Light, a substance, papa ! what, a thing 
that can be touched and handled ? 

Mr. P. — Material substances may exist which it is im- 
possible for you to handle. The air, for instance, is quite 
imperceptible to the touch,* though its materiality can be 
rendered evident to the senses in other ways ; yet air is 

* The air is always felt when there is a wind ; and it may be felt, at any 
time, if the hand be moved quickly through it.— Am. Ed. 



LIGHT. 129 

supposed to be more dense, compared with light, than 
quicksilver is, compared with air. The sun is the grand 
source of light, and it is imagined, by many, that the par- 
ticles of light are constantly emitted from that body, which 
must, consequently, be daily diminishing in size and 
brightness; but the more generally-received opinion is, 
that light exists throughout the universe as a separate elas- 
tic fluid, and that this fluid is brought into action by hav- 
ing a vibratory motion given to it, in some unknown man- 
ner, by luminous bodies. Respecting the nature of light, 
it must be acknowledged that we are in extreme ignorance ; 
yet many of its general properties are well understood, and 
it is with these I wish to make you acquainted. Wise as 
you seem to consider yourself on this subject, Robert, I 
will venture to say, you can scarcely answer a single ques- 
tion respecting it. Can you, in the first place, tell me how 
fast light travels ? 

Robert. — Light travels ! What is it you mean, father ? 

Harriet. — Why, Robert, you are puzzled at first start- 
ing : but do tell us, papa. 

Mr. P. — It has been ascertained that light travels, or is 
communicated, from the sun to the earth in seven minutes 
and a half, which is at the rate of 192,500 miles in a 
second of time. This is a degree of speed of which we 
can form no conception ; for a body moving at that rate 
would travel eight times round the world while you were 
counting " one." The transmission of light from objects 
on the earth's surface seems, therefore, to be instantaneous ; 
and an object is seen as soon as the light issues from it. 

Frederick. — Does the light from other bodies, then, 
travel as quickly as that from the sun ? 

Mr. P. — There is every reason to believe that it does. 

Harriet. — Do you mean, papa, such light as comes 
from the fire and from candles? 

Mr. P. — I mean, my dear, the light from all objects. 



130 LIGHT. 

Harriet. — What ! do trees, and fields, and houses, send 
out light ?* 

Mr. P. — Yes, Harriet : you would not see them if they 
did not ; nor should I now see you if every part of your 
face were not sending out rays of light to my eyes. The 
light, however, that proceeds from such objects, is not their 
own ; it is borrowed from the sun, or from other sources of 
light, and is reflected from their surfaces. The degree of 
this reflection varies according to the nature of the surfaces 
of the reflecting bodies, some of which reflect much more 
light than others. It is this difference in their powers of 
reflecting light that causes the difference in the brilliancy 
of objects. The greatest quantity of light is reflected from 
a finely-polished mirror, and the least from an unpolished 
black surface. 

Robert. — But the surface of a looking-glass, placed in 
the light of the sun, appears almost black, excepting when 
you look in the direction of the reflection. 

Mr. P. — It does so, Robert; and the more perfectly 
the light is reflected, the darker does the reflecting surface 
appear, when not viewed in the line of reflection. 

* An opaque body is one which neither shines with its own light nor 
permits light to pass through it. Trees, fields, houses, &c, are opaque 
bodies. 

Luminous bodies are those which shine with their own light : like the 
sun, a fire, or the flame of a lamp. 

Transparent bodies are such as permit the light to pass through them 
freely. Any thing which is perfectly transparent is invisible. Of course, no 
solid or liquid substance, with which we are acquainted, is perfectly trans- 
parent; but water, air, glass, and some of the gems, are nearly so. 

When bodies permit the light to pass through them, but in small quantities, 
they are said to be translucent. We have examples in alabaster, white 
glass, china ware, &c. 

Convergent rays are such as approach each other. 

The point where convergent rays meet, is called their focus. 

Divergent rays are those which separate more and more, the farther they 
go from the radiant point. — Am. Ed. 



LIGHT. 131 

Harriet.— But how can that be, papa, if, as you said 
just now, those things appear most brilliant that reflect the 
most light ? 

Mr. P. — The difficulty will vanish, I trust, on a little 
further investigation into the properties of light. You 
must understand that the rays of light, by means of which 
all objects are seen, are, of themselves, actually invisible, 
unless received directly upon the eye ; and that it is only 
when reflected directly to the eye that they become ap- 
parent. 

Harriet. — You seem determined to puzzle us this 
morning, papa; for the subject that Robert thought we 
knew so well appears to be the most difficult to understand. 

Robert. — If we see the light coming from the sun, as I 
do now, how can it be said to be invisible 1 

Mr. P. — I will try to convince you that it is so. Close 
the window shutters, Robert : there is a small hole in one 
of them, through which a ray of light will be admitted, on 
which to make our experiments. (Robert closes the 
shutters, and a beam of light from the sun shines in through 
the hole, and forms a bright spot on the wall.) The light 
from the sun is visible now that it is reflected from the wall : 
but you cannot see it, as it passes from the hole in the 
shutter to the wall. 

Robert. — Yes, father, I see a streak of light all the 
way. 

Mr. P. — I admit the direction of the light is percep- 
tible ; but what you see is not the direct ray of light itself; 
it is only the reflection of the light from the moats floating 
in the air of the room. If the air were perfectly transpa- 
rent, you would not see any appearance of light between 
the wall and the shutter; and, on examination, you will 
find the light proceeds from small particles floating in 
the air. 



132 LIGHT. 

Frederick. — Yes, I perceive it is so; the beam of 
light seems to be full of little moving things. 

Mr. P. — It is those that reflect the light, as it strikes 
against them, and make the course of the ray perceptible. 
I will now place a piece of white paper for the sun's 
beam to fall upon, and you will perceive that the light in 
the room will be much greater than it is when reflected 
from the dark-colored wall. (Mr. Powell places the paper 
against the wall.) 

Harriet. — Yes, I can now see where you all are, very 
distinctly, which I could not do before. 

Mr, P. — When I substitute a piece of black cloth for 
the white paper, a very different effect will be produced. 

Harriet. — The room seems to be now all in darkness — 
I cannot distinguish any thing. 

Frederick. — If you were to put a looking-glass in the 
ray, what would be the effect ? 

Mr. P. — We will try it. (Mr. Powell holds a small 
plane mirror in the sun's beam, and directs the reflected light 
first against the wall, and, then towards the hole in the shut- 
ter, through which the sun's rays enter.) You perceive, 
when the light is reflected against the wall, the effect is 
nearly the same as when the direct light of the sun falls 
upon it ; the spot of light appears rather less bright, be- 
cause the whole of the light is not reflected by the glass. 
Now, however, that I direct the reflected light back again 
into the sun's beam, the room is in darkness. In these 
variations of the experiment, the same quantity of light 
entered the room, and the difference you noticed in its 
effect upon surrounding objects, was produced by the differ- 
ence in the surfaces from which it was reflected. I trust, 
therefore, that you are satisfied that light is of itself invis- 
ible, excepting when received directly upon the eye, or is 
reflected to it from the objects of sight. 



LIGHT. 133 

Frederick. — But what was the reason, father, of the 
reflection from the white paper producing more light in the 
room than the reflection from the looking-glass? for I 
suppose it is not, really, so good a reflector as the glass. 

Mr. P. — No, my dear, it is not, as you might perceive 
from the bright spot produced by the reflection of the sun's 
light upon the wall ; but the more perfect the reflector ia, 
the more nearly does the light reflected resemble that 
which it reflects. The polished surface of the looking- 
glass being capable of reflecting most of the light thrown 
upon it, the reflected light had all the characters of the 
original rays, and did not become visible until again re- 
flected from some other surface, whence the rays could be 
diffused to all parts of the room. As nearly all the light 
was reflected in one direction, the surface of the glass 
appeared dark when viewed from any other point than that 
in which the rays were sent ; and if the mirror had been 
capable of reflecting all the light that fell upon it, it would 
have appeared quite black ; because no light would have 
been left upon the surface of the glass to send out rays in 
any other direction than that in which the whole of the 
light would be reflected. 

Frederick. — Then what is the cause, father, of the 
paper being visible in all parts of the room ? 

Mr. P.. — It is owing to the inequalities of its surface. 

When parallel rays of light are reflected from a perfectly 

flat, smooth, and highly-polished mirror, the reflected rays 

are also parallel, as represented in 

Fig. VII. this figure. Let a b c [Fig. VII.] 

d«/ represent three parallel rays of light 

J i ': / from a luminous body, striking against 

i I / the plane mirror g Ji, and thence re- 

s' i / & * 

° >m" j fleeted in the directions d ef, accord- 

^ikr^ ° ing to the general laws of reflection, 




134 LIGHT. 

which shall be afterwards explained to you. These reflected 
rays, it will be seen, are parallel to one another, and would 
be visible only to an eye placed in the direction to which 
they are reflected ; while an eye looking 
at the glass from the point p would see 
no light whatever. But when the rays 
of light fall upon an uneven surface, 
the effect is very different. Suppose a 
bed [Fig. VIII.] to be parallel rays 
of light falling upon a polished angular 
surface, k I; some of the rays would 
be reflected upwards towards e f, and 
some in the opposite direction g h; 
and the result of reflection from a number of such angular 
surfaces would be the diffusion of the rays of light in 
all directions; as represented in this figure, [Fig. IX.] 

in which the parallel horizontal 

Fig. IX. lines are supposed to be rays, 

J / . , proceeding from a luminous 

object, striking upon the an- 

^32 gular surfaces of the body a b; 

ZIZZZ and the dotted lines represent 

1 the varied directions into 

V^r-> N which the light would be 

rV \\ reflected. The brilliancy of 

\X \\ some kinds of spars is owing 

N "" to the numerous crystals of 

which their surfaces are com- 
posed, by the disposition of which the rays of light are 
reflected to the eye from several of their polished angles 
at the same time. 

Frederick. — I suppose, then, father, that the objects in 
a landscape have the power of diffusing the rays of light 
in ull directions, or else they would not be- visible from 
every point of view, as they are. 



LIGHT. 135 

Mr. P. — Exactly so. Most of their surfaces are suffi- 
ciently rough to spread the light all around them. 

Harriet. — On a cloudy day, papa, when the sun cannot 
shine upon any thing, where does the light come from, that 
is reflected in this way ? 

Mr. P. — The clouds, it is true, obscure a great part of 
the sun's light ; but they are never so dense as to obstruct 
it altogether. The light of the sun, when it strikes upon 
the particles of moisture forming the clouds, is diffused 
through their whole mass ; therefore, the light we receive 
on cloudy days, instead of coming in parallel rays directly 
from the sun, is diffused among the vapors in the air, 
which thus become a great reservoir of light, and transmit 
it to the earth in various directions. Even on the clearest 
day, a great portion of the light from the sun is diffused by 
the vapors of the atmosphere. It is this diffusion of the light 
that produces the bright appearance of the sky. Were the 
air to be perfectly transparent, the sky would appear almost 
black ; because, as the rays of light are invisible, excepting 
when they strike directly upon the eye, if there were noth- 
ing above us that could reflect them, no light could be 
perceived, and the sun himself would appear like a brilliant 
orb surrounded by the darkness of night. 

Frederick. — In very clear weather, then, I should 
suppose the sky must appear darker than it does when the 
atmosphere is full of vapor. 

Mr. P. — You are right, Frederick. In a fine dry 
climate, the sky is of a much deeper blue than we ever 
behold it in this country ; and at the tops of high moun- 
tains, above the misty exhalations of the earth, the sky 
appears of a, still deeper color. It is to the diffusion of light, 
by the vapors of the atmosphere, that we are indebted for 
the twilight that ushers in the day, and cheers its departure. 
In a perfectly transparent atmosphere, we should be left in 



13b LIGHT. 

darkness the instant the sun was set ; but the clouds and 
vapors reflect the sun's diffused light long after he is below 
the horizon, and, during the summer months, spread a 
genial twilight throughout the night.* 

* For the twilight we are indebted, partly to the cause assigned in the 
text, and partly to the refractive power of the atmosphere, which will be 
explained hereafter. If the earth had no atmosphere, we should be left in 
total darkness the moment the sun set, and the heavens would be dark, 
even at noon-day 5 but these things would not occur, if the atmosphere were 
transparent, unless its refractive power depends upon the vapors contained 
in it,— which some have supposed to be the fact. 

The duration of twilight varies in different latitudes, and in the same 
latitude at different seasons of the year. It is shortest in the equatorial, and 
longest in the polar, regions. It does not continue through the whole night, 
at any season of the year, until we reach about the fiftieth parallel of lati- 
tude j and there only when the days are the longest. There is no part of 
the United States, where the morning twilight begins before the evening 
twilight ends. 

The duration of twilight depends upon the height of the atmosphere, on the 
quantity of matter it contains capable of, reflecting light, and on the angle 
which the sun's course makes with the horizon ; for the more oblique thi s 
is, the longer will twilight continue. Now, as the actual height of the at- 
mosphere and the quantity of moisture it contains, depend, in some degree 
upon its temperature, it is evident that the duration of twilight should vary, 
on this account, with the different seasons. Because the atmosphere is 
dilated and filled with vapors by the heat of the day, the evening twilight 
is longer than that of the morning, other things being equal.— Am. Ed. 



QUESTIONS. 



1. What is light supposed to be ? 

2. What is the grand source of light ? 

3. How fast does light travel ? 

4. In what way is this velocity illustrated ? 

5. Does light from other bodies travel as fast as light from 
the sun ? 



LIGHT. 137 

6. What is an opaque body ? — Luminous ? — Transparent ? — 
Translucent ? 

7. What are divergent rays ? — Convergent ? — What is a focus ? 

8. By what light do we see opaque bodies ? 

9. What surfaces reflect the most light ? — What the least ? 

10. Why does a looking-glass appear quite black in the light of 
the sun ? 

11. Are rays of light visible ? 

12. By what experiment is this shown ? 

13. What is the effect when the beam of light is received on a 
piece of white paper ? — Black cloth ? — Looking-glass ? 

14. Explain these results by Figures 7, 8, and 9. 

15. How do you account for the sparkling appearance of waves in 
the sunshine ? 

16. Why is it not perfectly dark when the rays of the sun are 
intercepted by clouds ? 

17. If the air were perfectly transparent, what would be the 
appearance of the atmosphere ? 

18. How does the air appear in a very dry climate ? — On the tops 
of high mountains ? 

19. By what is the morning and evening twilight produced ? 

20. Where is the twilight longest ? — Where shortest ? 

21. Does twilight continue all night in any part of the United 
States ? 

22. Can you give any reason why the sun should appear less 
bright when it sets than at noon ? 

23. What causes the bright line of light on water, or a field 
of snow, when the moon shines ? 

24. Why is it difficult to light rooms well, which are covered 
with dark paper ? 

12* 



ltf© liUiHT. 



CONVERSATION XV 

LIGHT (continued.) 



Robert. — By what means, father, are we able to 
distinguish any thing on a dark, cloudy night, when no 
light can reach objects, to be reflected from them 1 

Mr. P. — In the first place, Robert, I cannot admit the 
truth of your premises. There never was a night so 
dark as to be totally devoid of light. Indeed, it may 
be doubted whether light could, under any circumstances, 
be absolutely extinguished ; or, at all events, our senses 
will not enable us to say when there is no light. 

Robert. — Not such a thing as darkness, father ! What 
light can there possibly be in a room with the shutters 
and door closed, on a dark night, when there is neither 
candle nor fire 1 

Mr. P. — I cannot pretend to tell what quantity of 
light there may be in such a room ; but that there is some 
light I may venture to affirm, though our eyes cannot 
perceive it. A certain quantity of light is requisite to 
enable us to distinguish the forms of objects, and a still 
greater to distinguish their colors. The absence of the 
smallest of these quantities we are accustomed to term a 
state of total darkness ; yet, to other organs of vision, 
more delicately formed, this total darkness may seem as 
brilliant as daylight. The eyes of beetles and mice, 



LIGHT. 139 

and of other creatures that make night their time of 
action, we must suppose to be so constructed as to 
enable them to see objects distinctly, when they are to us 
invisible. Even the eyes of men, when they have been 
immured in the darkest dungeons for a number of years, 
have become so sensitive as to distinguish all the objects 
in their dismal abodes. An eye so accustomed to dark- 
ness suffers intolerable pain when again exposed to the 
light of day ; and even in a dark night objects would 
appear perfectly distinct.* 

Frederick. — Then what we call darkness is only a 
diminished quantity of light ; in the same manner as you 
explained cold to be but a comparative diminution 
of heat ? 

Mr. P. — You are right, Frederick. We judge of light 
and darkness by comparison ; and what appears to be 
light one moment, may appear as shadow the next, if a 
brighter light be contrasted with it. Even the flame of 
a candle may be made to appear as a shadow against the 
wall by the light of a brilliant lamp. 

Harriet. — What makes a shadow against the wall 
seem so much larger when any thing is held close to the 
candle than when it is held near the wall 1 

Mr. P. — The rays of light diverge from the flame of 

* Light enters the eye through the pupil, which is a round hole in the 
middle of the iris, or the colored part of the eye. The pupil is so constructed 
that it readily adapts itself to the degree of light in which it is placed. When 
the light is faint, the pupil dilates so as to admit an additional quantity of 
rays j and when it is bright, the pupil contracts, that no more rays may be 
admitted, than are necessary for distinct vision. It has been estimated 
that the pupil of the human eye, when most dilated, is ten times as large, as 
when it is most contracted. The difference is vastly greater in cats, owls, 
mice, and other animals, that are abroad in the night. If you look towards 
the sun, the pupils of your eyes become contracted almost to points ; if you 
then cover your eyes so as to exclude the light, the pupils will expand 
beyond their ordinary dimensions.— Am. Ed, 



14U LIGHT. 

a candle in straight lines in all directions, in a similar 
manner to lines drawn from the centre of a circle to its 
circumference. If, therefore, you place an object near 
the flame, it receives more of the diverging rays, and, 
consequently, obstructs more of the light ; and a greater 
space of the wall appears in darkness than when it is held 
nearer to the wall. That this must be the effect will be 
rendered evident by this drawing ; in which a represents 

Fig. X. 




the candle, and the diverging lines the rays of light 
issuing from it towards the wall. These rays, if there 
were no intervening object, would strike against the wall 
and illuminate the space between b and c. But when an 
opaque object is placed near the candle, as at d, it will 
receive the whole of these rays, and the space between b 
and c will be in darkness ; or, in other words, the shadow 
of the object d will appear against the wall of the size of 
b c. When the same object is removed farther from the 
candle, as at h, many of the rays it before obstructed will 
pass over and under it, and illuminate parts of the wall 
that were before darkened by its shadow, and the shadow 
will now be diminished to the size of f g. 

Robert. — Then is there as much light upon a small 
object, when placed near to the candle, as there is upon 
the large space on the wall covered by its shadow ? 

Mu. P. — Yes, Robert, there is. Light, when ema- 
nating from a point, — as the flame of a candle may be 
considered — by diverging as it expands, diminishes in 



intensity in proportion to the space it illuminates. The 
diverging rays of light are known to diminish in intensity 
as the squares of the distance increase : that is, if an 
object be removed twice its distance from a candle, it will 
receive only one fourth the quantity of light ; and if 
removed to four times the distance, it will receive only 
one sixteenth part of the light. As it is of importance 
you should understand how this diminution of light is 
caused, I will make the subject more clear by a drawing. 

Fig. XL 



,U-L 
TT 



Here we have again a representation of the flame of a 
candle, a, sending out its diverging rays of light ; and 
the squares bed we will suppose to be three square 
screens. The first one, b, we will imagine to be one inch 
square, the second to contain four square inches, and the 
third sixteen. Let b be placed at a distance of one foot 
from the candle, c two feet from it, and d four feet. 
Now, it is evident that the first screen, so placed, must 
obstruct all the diverging rays on their passage to the 
second, and prevent any light from falling upon it ; that 
is, b will receive all the light which, were that screen not 
there, would come to the share of c. In the same 
manner the second screen, c, would rob d of all light 
from the candle, and it would receive on its surface, of 
four square inches, the same quantity of light which, if it 
passed on to d, would be spread over the surface of 
sixteen square inches. But if the first screen, which is 
only one inch square, receive as much light as the second, 



142 LIGHT. 

whose surface is four times as large, the light on the first 
must be four times as great as it is on the second screen, 
and sixteen times more intense than upon the third screen, 
on which the same quantity of light is spread over sixteen 
times the surface. The light of a candle would, in this 
manner, continue to diminish, till it would become at 
last invisible. 

Harriet. — How far will the light of a candle 
reach, papa ? 

Mr. P. — It can be seen, on a clear night, at a distance 
of two miles ; but there are no limits to the distance to 
which its light will really extend. If we allow the inhabit- 
ants of the moon to possess organs of sight sufficiently deli- 
cate to be sensible to the impression of light so attenuated, 
we may imagine them to be able to read by the light of a 
candle burning, on a clear night, upon the surface 
of the earth. 

Harriet. — What a very droll idea ! But if they could 
see the candle, they might see us, I suppose ? 

Mr. P. — One supposition, my dear, is just as reasonable 
as the other. But, in point of fact, as we know of 
nothing that should totally obstruct the light of a candle 
on its course to the moon, we may reasonably conclude 
that its rays form a minute portion of the general mass 
of light which that heavenly body must receive from 
the earth. 

Harriet. — Does the moon receive light from the earth, 
as we do from the moon 1 

Mr. P. — Yes, my dear ; and our globe must appear to 
the inhabitants of the moon, if inhabitants there be, a 
much larger and more brilliant orb than the moon does 
to us, in consequence of the earth being so much larger 
than the moon. 

Harriet. — But does the earth shine, as the moon does? 



LIGHT. I4:j 

Mr. P. — Exactly in the same way — with light bor- 
rowed from the sun. The light we receive from the 
moon is not produced by any luminous property in that 
body, but is merely the light of the sun reflected to the 
earth from all the objects on its surface. 

Robert. — But what can make them shine so ? for our 
houses, and trees, and hills, don't shine. 

Mr. P. — Yes, they do, or you would not see them. 
Every thing that sends out rays of light to the eye may be 
said to shine : but as their reflected light is dull, when 
compared with the light of the sun, we do not consider 
them as shining bodies. Even the moon ceases to shine 
after the sun has risen, and appears only as a circular 
white body, though she is then reflecting as much light to 
the earth as during the night, when she appeared 
so brilliant. 

Robert. — But, father, you told us that light from all 
objects diminished four times whenever the distance was 
doubled, and that a screen, placed four feet from a candle, 
received only a sixteenth part of the light, as the same 
screen when placed one foot from it ; then how can the 
light, reflected from the things on the face of the moon, 
appear so bright at such a great distance ? 

Mr. P. — If you look again at this drawing of the 
screens and candle, it will explain the difficulty. In this 
drawing, you perceive that the screen of one square inch 
receives as much light as the screen containing sixteen 
square inches ; but, as the quantity of light is the same 
in both cases, the same quantity will be reflected from the 
large screen as from the small one, and it will make up 
in quantity what it wants in intensity. The same will apply 
to the moon. If we could approach within about two 
hundred and fifty yards of the moon, a circle of one yard 
diameter on her surface would then appear as large as 



144 LIGHT. 

does her whole disc, which is two thousand one hundred 
and eighty miles in diameter, when viewed from the 
earth; and as much light would be given out by that 
small circle, at that distance, as we receive from the 
whole orb. As we receded from the moon towards the 
earth, every time that we double the distance it would 
require a circle of twice the diameter, (therefore four 
times the surface,) to equal the apparent size of the moon's 
disc on the earth ; and when we arrived at our globe, the 
whole surface of the moon would appear of the same size, 
and reflect the same quantity of light, as a circle of one 
yard diameter did at a distance of only two hundred 
and fifty yards. 

Frederick. — Would a circle of a yard diameter on 
the earth give as much light, at a distance of two hundred 
and fifty yards, as the whole moon ? 

Mr. P. — Yes ; a white circular board of one yard 
diameter, placed two hundred and fifty yards off, so as to 
reflect the direct light of the sun shining on it, would 
afford more light, at that distance, than is given out by 
the moon. 

Harriet-. — You don't mean that it would shine as the 
moon does? 

Mr. P. — Yes, my dear, if we could see it, as we see 
the moon, when all other objects are in darkness. 

Robert. — I can't see how that can be, father ; for the 
moon, on a clear night, seems to give nearly as much 
light as the sun. 

Mr. P. — It is owing to the contrast between the light 
and darkness, that makes you think the moon so bright. 
It has been ascertained that the light from the full moon 
is only about one hundred thousandth part of that derived 
from the sun at noonday. 



Harriet. — What is it that makes the man's face 
in the moon ? 

Mr. P. — The shadows on the moon's disc, that have 
been thought to bear some resemblance to the features of 
a man, are produced by the unequal reflection of the 
sun's rays from the earth and water on the surface of the -^ 
moon ; the darker parts are supposed to be the reflection 
from water, and the bright ones the reflection from land.* 

* It is the opinion of many astronomers, thai there are neither seas nor 
lakes in the moon ; in a word, that it contains no water. Its surface is very 
much diversified by stupendous mountains, deep caverns and extensive 
plains. The different degrees of illumination which it exhibits, are probably 
occasioned by these remarkable inequalities of its surface.— Am. Ed. 



QUESTIONS. 



1. Can objects be distinguished by the eye in total darkness? 

2. What is darkness ? 

3. What is said of the eyes of certain animals ? 

4. Does the same degree of light appear equally bright to us 
under all circumstances? — Explain the difference. 

5. How is light admitted into the eye ? 

G. How do you account for snotc blindness ? 

7. Why is the light painful, when you first open your eyes in the 
morning ? 

8. In what way do we judge of light and darkness ? 

9. Why is a shadow on the wall larger, when an opaque object 
is held near the candle, than when it is held near the wall ? — Ex- 
plain Figure 10. 

10. By what law does the intensity of light diminish ? 

11. Explain Figure 11. 

12. How far will the light of a candle reach ? 

13. How does the earth appear to the inhabitants of the moon ? 

14. By what light do the earth and moon shine ? 

15. How is the apparent brightness of the moon explained ? 

13 



146 LIGHT. 

16. How much does the light of the sun, at noon, exceed that of 
the full moon ? 

17. Why is it that all parts of the moon do not appear equally 
bright ? 

18. Why are more lights necessary in a large hall than in a small 
room ? 

19. What is the appearance of the moon in the day-time ? 

20. Why does it appear less bright than in the night ? 

21. In what manner do the rays of light, emitted from a lumi- 
nous body, move ? 



CONVERSATION XVI 



REFRACTION OF LIGHT. 



Mr. P. — We will now consider another very important 
property of light, namely, its refraction. 

Frederick. — What is it you mean, father, by the 
refraction of light ? 

Mr. P. — The rays of light proceed from luminous 
objects in straight lines, and so continue as long as the 
transparent medium through which they pass remains the 
same; but when a ray of light enters obliquely into a 
different medium, (that is, if, after passing through air, it 
enters into glass or water, or after passing through the 
latter it enters into air,) it is bent from the direction of 
its former course ; and this bending of the rays of light is 
termed its refraction. 

Frederick. — Do the rays, after they have been bent in 
this manner, continue in a straight line in their new direction? 
Mr. P. — Yes, while they pass through the same me- 
dium ; but I shall make the 
subject more clear to you by 
a drawing. Let g e f be a basin 
or glass full of water, and a b 
a ray of light striking against 
the surface at b, in the oblique 
direction a b. Its natural straight 
course, if the vessel were empty, 




148 REFRACTION OF LIGHT. 

would carry the ray to c ; but as soon as it enters the water, 
it is bent in a more perpendicular direction, and arrives 
at the bottom of the vessel at the point e, in the direction 
from b to e. 

Frederick. — When light is refracted in this manner, is 
it always bent more perpendicularly 1 

Mr. P. — If the light pass from a rarer medium into one 
more dense, as from air into water, it is always refracted 
in a direction nearer to the perpendicular of the surface of 
the denser body. Thus, supposing a b to be a ray of light 
striking against a thick piece of plain 
glass, d e, (the perpendicular to whose 
surfaces is p r,) as soon as the light enters 
the glass, it will be refracted in the direc- 
tion marked by the line b g, which is 
nearer to the perpendicular of the surface 
of the glass than b c, its former direction. 
When a ray of light issues from a denser 
medium, on the contrary, the refraction 
takes place in a direction farther from the 
perpendicular of the surface. Thus, in the figure before 
us, the ray b g, on issuing from the glass into the air, will 
be inclined towards h, and will be thus bent as much from 
the perpendicular, on coming again into the air, as it was 
bent towards it on entering the glass. I will now make 
the refraction of light visible to you by experiments. 

Harriet. — I shall be very glad of that, papa, for I am 
almost tired of looking at straight lines and a b c. 

Mr. P. — I will show you, first, the experiment I exempli- 
fied in our drawing of the vessel of water and the ray of 
light ; to do which, we must close the shutters, and admit 
the sun's light through the aperture only. (Robert closes 
the shutters, Mr. Powell having first procured a large 
glass tumbler and a jug of water ; and when the shutters 




are closed, he places the glass on the table in the direction 
of the sun's beam.) You perceive the rays of light 
enter the glass in an oblique direction, and pass through 
its side a little above the bottom. I will fill it with water, 
slightly tinged with color to enable you to see more 
distinctly the direction of the ray through the fluid. 
Where is the spot of light now, Robert ] 

Robert. — Quite at the bottom of the glass. 

Mr. P. — Look through the water, and observe where 
the light is bent by the refraction. 

Robert. — I see, that just where the beam of light 
enters the water, it appears to be broken and bent down. 

Harriet. — [Looking through the water.) It is very 
curious : what makes it do so, papa ? 

Mr. P. — The cause of refraction has not been discov- 
ersd, my dear ; for no attempt yet made to explain the 
phenomenon seems quite satisfactory. We are acquainted, 
however, with the laws which govern refraction, and are 
enabled to apply them to very important uses. 

Harriet. — What use can be made of refraction, papa? 

Mr. P. — If light were not subject to refraction, we 
should not be able to make telescopes, microscopes, 
spectacles, nor any other optical instruments. The power 
of convex lenses, or magnifying glasses, depends entirely 
upon this property of light. So great is its importance, 
indeed, that did it not exist, we should not be able to see ; 
and the beautiful machinery of the eye would be useless, 
if the rays that enter it were not refracted by its various 
lenses. 

Frederick. — Can you explain to us, father, how the 
effect of magnifying glasses is produced by refraction 1 

Mr. P. — I will do so at a future opportunity ; but we 
shall now confine our attention to the effects of refraction, 
as exhibited to us in the natural phenomena around us. , 
13* 



150 REFRACTION OF LIGHT. 

Robert. — Is light refracted in the same degree, on 
passing through all transparent substances? 

Mr. P. — No, my dear : the degree of its refraction 
varies according to the difference in the densities between 
the refracting medium and that through which the light 
first passes. Light, in passing from air into water, is 
refracted one fourth nearer to the perpendicular than its 
former direction ; and in passing from air into glass, it is 
refracted one third nearer. The rays of light, in passing 
from a vacuum into air, are also refracted from their 
original course ; and it has been discovered that the light 
from the sun, and from the other heavenly bodies, is thus 
refracted on coming to the earth, and that we consequently 
never see them in their true positions in the heavens.* 
Owing to this refractive power of the atmosphere, we see 
the sun several minutes before his direct rays would reach 
the eye ; and his image, from the same cause, remains in 
sight some time after he is really sunk below the horizon. 

Robert. — How can that be, father ? 

Mr. P. — The experiment I have just shown you will 
explain the matter clearly. The beam of the sun's light, 
you observed, passed through the side of the glass before 
it was filled with water, and w T ould, therefore, not be 
perceptible to an eye placed at the bottom, until the sun 
rose higher in the heavens, and threw his beams directly 
on the bottom of the glass. But when the water was 
poured in, the rays were bent down to the bottom, and 
the sun would become visible from that point. The 
water, in this case, represents the effect of the atmosphere 
of our earth, which thus draws down the rays of the sun's 

* This is Irue of every heavenly body not in the zenith; that is, not 
directly above us. The refraction is greatest when the body is in oe near to 
the horizon, and decreases as it ascrnds. Hence, when the moon first rises, 
the light from its lower side is more refracted than that frost its upper side. — 
Am. Ed. 



IVul IV.IU 1 HJil V/l' 1-llUlll 



light from their original course, and renders him visible 
before his rays would otherwise reach the eye. 

Frederick. — Do we see the sun and the stars in their 
true position, or do they seem to be in the direction in 
which their rays are refracted to us 1 

Mr. P. — All objects appear to be situated in the 
direction from which the rays proceed to the eye ; 
therefore, the sun and the stars appear to us to be higher 
in the heavens than they really are. We will now reverse 
the order of our experiment, and let the light proceed 
from an object at the bottom of the basin, through the 
water, into the air. An eye placed so as to look into the 
glass in the direction that the sun's beam entered it, will 
not, while it is empty, see any object at the bottom ; but 
when it is filled with water, the bottom of the glass will 
appear to be raised, and an object placed there will 
become visible, 

Harriet. — I should like to see that very much. 

Mr. P. — Very well, Harriet, you shall soon be satisfied . 
it will be better to substitute a basin for the glass, else you 
would see through the sides, and the effect would be 
destroyed. [Mr. Powell fixes a wafer at the bottom of 
a white basin, and Harriet stands at such a distance 
from it as to lose sight of the bottom. Mr. Powell then 
fills the basin gently with -water.) 

Harriet. — As you pour in the water, I see the bottom 
part of the basin gradually rising — and now I see the 
wafer ; it appears nearer to the top than it was. 

Mr. P. — You may learn from this experiment that 
rivers and ponds are really deeper than they seem to be, 
owing to the refraction of the light proceeding from the 
objects at the bottom ; which, therefore, appear to be 
nearer to the surface than they are. From the same 
cause, objects in water appear not only different in 



152 REFRACTION OF LIGHT. 

position, but different in shape. I dare say you have 
often observed that when you put a stick into water, it 
seems to be bent as if it were broken. 

Frederick. — Yes, I have very often, and it has always 
puzzled me. I shall know how to account for it in future, 
by the refraction of the rays of light as they proceed from 
the stick through the water into the air. 

Mr. P. — Very good. You may repeat the experiment 
with the basin of water and the wafer by yourselves : it is 
a striking illustration of the effects of refraction. The 
experiment may be varied, by fixing two or more wafers 
at the bottom, so that they may become visible one after 
the other. 



QUESTIONS. 

1. How do rays of light move ? 

2. What do you understand by a medium ? 

3. What is meant by the refraction of light ? 

4. Are rays of light, which pass perpendicularly from one medium 
into another, refracted ? 

5. How do they move after refraction ? 

6. How is light refracted when it passes from a rarer into a 
denser medium ? 

7. Explain Figure 12. 

8. When a ray of light passes from a denser to a rarer medium, 
how is it refracted ? 

9. Explain Figure 13. 

10. Give an account of the experiment, which proves that light, 
in passing from a rarer into a denser medium, is refracted towards a 
line perpendicular to the surface of the latter. 

11. Is the cause of refraction known? 

12. Is light refracted in the same degree, in passing through all 
transparent substances ? 

13. Is light refracted in passing from a vacuum into the air ? — In 
what way ? 



REFRACTION OF LIGHT. 153 

14. What effect has the refractive power of the air upon the appa- 
rent positions of the heavenly bodies? 

15. How does it affect the length of the day ? 
1G. In what manner do you explain this fact ? 

17. In what direction do we see objects ? 

18. By what experiment is the answer to question eighth 
illustrated ? 

19. Why does a spoon in a tumbler of water seem bent.'' 

20. When you look from the shore into a river or pond, do you 
see its real depth ? — Why not ? 

"21 . When you look down into water from a boat, do you -see its 
real depth ?— Why so ? 

22. To what particular uses is the refraction of light applied ? 

23. Why does a post, standing obliquely in the water, appear to 
be bent ? 

24. Can you account for the oval appearance, which the moon 
sometimes exhibits, when near the horizon ? 

25. Is light refracted by glass windows ? — In what way ? 

28. Do the rays of the sun, or of any other heavenly body, move 
through the air in straight or curved lines ? 

27. How much is light refracted, in passing from air into water ? 

28. How much, in passing from air into glass ? 



154 COLORS. 



CONVERSATION XVII 



COLORS. 



Robert. — You told us, father, the other day, that the 
light from such objects as trees and houses, is the light of 
the sun reflected from them. But I have been thinking 
that it must be something else besides the light of the sun ; 
for almost every thing we look at is of a different color, 
and the sun's light is of no color at all. 

Mr. P. — The difficulty you have started, Robert, evinces 
a reflective and philosophic mind, which I trust you will 
cultivate to advantage. It was taken for granted, previous 
to Sir Isaac Newton's discoveries on the subject, that light 
was, as you conceive it to be, without color ; and several 
curious explanations were attempted by ancient philoso- 
phers to account for the color of bodies. Some imagined it 
to be a flame issuing from them, while others affirmed that 
the different colors were produced by different motions 
communicated to the particles of light. We are indebted 
to Sir Isaac Newton for the discovery, that the light which 
emanates from the sun, though apparently so colorless, is 
really composed of all the colors mingled together. 

Robert. — What ! is the white light of the sun full of 
colors ? I cannot think how it is possible. 

Mr. P. — If we admit the sun's beam through the aper- 
ture in the shutter, as in our former experiments, I can 



COLORS. io5 

separate the colors of which it is composed, and show them 
to you. 

Harriet. — I am quite anxious to see how you can do 
so. (Frederick and Robert close the shutters, and 
Mr. Powell, having provided himself with a glass prism, 
places a sheet of white paper against the wall to receive the 
rays of light.) 

Mr. P. — This long triangular piece of glass is called a 
prism ; and its angular shape enables it to refract the rays 
of light passing through it very far out of their original 
course. When I place the prism so as to intercept the 
light on its passage to the screen, the round spot of light 
will become oblong, and be tinged with brilliant colors. 

Harriet. — The white light is gone ; but, higher up the 
screen, I see all the colors of the rainbow. 

Mr. P. — What you now see is the same bright light 
dissected into the different parts of which it is composed. 
The color at the bottom is red, the next to it is orange, 
above that is yellow, and then come green, blue, indigo, 
and violet, in succession, above one another. Sir Isaac 
Newton concluded, from this and from other experiments, 
that the light of the sun is composed of a mixture of col- 
ored rays, some of which are more easily refracted than 
others ; and that when a beam of light is very much bent, 
as is the case when it passes through a prism, the rays 
most refrangible are separated from those that are less so. 
The white light is thus divided, and the several colors of 
which it is composed are exhibited separately. The image 
of the sun's light, so dissected, is termed the solar spectrum. 

Robert. — Do you mean, father, that if all those colors 
I now see were mixed together, they would become white ? 

Mr. P. — Yes. When all the colored rays are collected 
together by a convex lens or burning-glass, the light will be 
colorless. Observe, now that I hold this lens between the 



15G COLORS. 

prism and the paper, at a proper distance, the colored rays 
are brought to a point, and appear as a bright spot of pure 
white light. 

Robert. — I am quite surprised to see how all the colors 
have disappeared. 

Mr. P. — The same effect may be produced by painting 
the colors of the solar spectrum, in their proper proportions, 
on the upper part of a top ; and when it is put in rapid mo- 
tion, the colors will be blended together and appear white.* 

Harriet. — How many colors are there in light? 

Mr. P. — The prism, you perceive, divides it into seven, 
of which red is the least refracted, and violet the most. 
Four of the seven, however, may be considered as merely 
mixtures of the other three, and the real primitive colors 
are red, yellow, and blue. With these, in different shades 
and proportions, all colors may be made.t 

Frederick. — I do not yet understand what causes the 
sun's light, when it is reflected from an object, such as a 
house, to appear of a different color from the light that 
strikes against it. 

Mr. P. — It is supposed that, owing to some peculiar 
disposition of the particles composing different substances, 
they possess the power of separating the rays of light into 
its component colors. Some of the colors are then ab- 
sorbed by the substance against which the light strikes ; 

* The white thus produced has a dingy appearance, because colors suffi- 
ciently pure cannot be used. A similar result may be obtained if we mix 
tog-ether seven fine powders, in proper proportions, having the colors of the 
prismatic rays.— Am. Ed. 

+ Philosophers have had different opinions as to the number of primitive 
colors. Sir Isaac Newton, who is generally followed, held that the seven 
colors seen in the solar spectrum, are all primitive. The orange may be 
produced by a union of yellow and red 3 and the green by a mixture of the 
contiguous rays, blue and yellow. But it would be difficult to obtain the 
other rays of the spectrum by any combination of the three colors named in 
the text. — Am. Ed. 



COLORS. 157 

and those which it has not the power of absorbing, are 
reflected from its surface. Thus, a brick house is said to 
absorb all rays excepting the red ; the red one it reflects 
to the eye, and we, therefore, call such an object red. 
Trees and fields, again, reflect the green rays (which are 
a mixture of the blue and yellow), and absorb the red. 

Frederick. — As the colors of the rainbow are so much 
like those produced by the prism, I suppose they are formed 
in the same manner, by the separation of the rays of the 
sun's light. 

Mr. P. — Yes, my dear, they are. When rain-drops are 
falling from the clouds, at the time the sun is shining upon 
them, part of the sun's light is reflected from the back of 
the drops to the earth. 

Robert. — Reflected from the transparent drops of rain, 
father ? 

Mr. P. — Yes, Robert, reflected. No substance is suffi- 
ciently transparent to permit all the light to pass through 
it ; and when light strikes very obliquely against glass or 
water, nearly the whole of it is reflected, and none trans- 
mitted. The reflection of the sun's light from a window, 
and from a glass of water on the table, are instances of 
partial reflection from the surfaces of transparent bodies, 

Frederick. — A drop of water, too, looks bright and 
sparkling in almost any direction ; is not that owing to its 
reflecting the rays striking upon it 1 

Mr. P. — It is a very appropriate illustration, Frederick. 
If the drops of rain that reflect the colored bow were flat, 
instead of being round, the light would be reflected from 
them to the earth, without being divided into its prismatic 
colors, and the rainbow would appear as a glittering arch 
of pure light : but the drops being circular, the inclinations 
of the surfaces at which the light enters, and from which 
it is afterwards reflected, are so oblique, that they bend 
14 




158 COLORS. 

the light so much as to separate them ; and it is, therefore, 
reflected to us in its variegated colors. Suppose, for 

instance, a b [Fig. XIV.] to 
r ig. XIV. b e a rav f light entering a 

rain-drop; it would be re- 
fracted to c, and part of the 
light would pass out of the 
drop towards d; but as it 
strikes at so oblique an an- 
gle, the greater portion of 
the light would be reflected 
to e. On re-entering the 
air, the ray would undergo another refraction, and be 
separated into its different colors; the red rays being at 
the bottom, and the violet at the top, as here represented. 

Robert. — But if the colored rays are spread so much, 
how can the eye take them all in at once ? 

Mr. P. — You forget, Robert, that there are drops of 
rain following each other, in rapid succession, from the 
clouds to the earth ; and that this refraction and reflection 
are going on in each. If, therefore, the eye were so placed 
as to receive the red ray from this drop, it would receive 
the orange ray from the drop below, and the yellow from 
the drop below that, and so on, until it received all the 
colors of the rainbow from successive drops. 

Robert. — Then, if the light come from so many falling 
drops, a rainbow ought to be always sparkling and glit- 
tering, like a moving glass lustre. 

Mr. P.— And so it most probably would, if the refraction 
took place in large drops of rain. Though I have been 
obliged, for the sake of clearness, to speak as if each color 
were produced by a single drop ; yet, as the drops of rain 
from which the light is refracted are at a considerable 
height, the drops must be there very small, resembling, per- 



COLORS. 159 

haps, the minute particles of water visible in a mist. The 
depth of each stratum of color in the rainbow, therefore, as 
well as throughout the extent of the arch, may be the result 
of refraction and reflection from numerous very small drops 
of rain, the rays from which become so blended together as 
to produce uniform color. In this manner we may con- 
ceive that the seven prismatic colors seen in the rainbow 
may proceed from hundreds of minute drops, above one 
another, instead of from only seven, as frequently supposed. 

There is sometimes a second bow formed above the first, 
in which the colors are reversed. This bow is produced 
by the refractions and reflections of rays entering the lower 
part of the drops. These rays are twice reflected at the 
back of each drop before they issue from it, when they are 
again refracted into the prismatic colors. In consequence 
of the light lost by the two reflections, this second bow is 
generally very faint. 

Frederick. — Is the halo that is often seen round the 
moon produced in the same way as the rainbow 1 

Mr. P. — Yes. The light of the moon, shining upon 
the condensed vapors of the atmosphere, will, under certain 
circumstances, be so much refracted as to separate into its 
prismatic colors, and form a beautiful circle, through the 
centre of which the direct rays of that heavenly body per^ 
etrate with unsullied brightness to the earth. 

Robert.— You told us, father, that all the colors were 
contained in light, but you have said nothing about black. 

Mr. P. — I thought you would have understood, from 
what had been previously said, that black is merely the 
absence of light ; the difference between black and white 
is, that white reflects all the rays united, and black absorbs 
them all, and reflects none. 

Robert.-— Then, if black do not reflect any light, how 
can it be seen by the eye 1 



160 COLORS. 

Mr. P. — By its contrast with surrounding objects, from 
which light is reflected. If a black body be placed on a 
white ground, for instance, the eye receives light from all 
parts immediately around it, and the absence of light from 
the black object renders its outline even more distinctly 
marked than if it sent out rays. It is in the same manner 
we distinguish a shadow. If you hold your hand between 
the candle and the wall, you perceive a distinct outline of 
the hand upon it, which is marked by the comparative 
absence of light on those parts of the wall where the light 
of the candle is intercepted ; and the outline is more dis- 
tinct than it would be if the form of the hand were painted 
in any transparent color upon glass, and held in the same 
situation, because the contrast between the light and com- 
parative darkness is greater than between the light and the 
color which sends rays to the eye. 

Robert. — Then black, which seems the strongest of all 
colors, is no color at all ! 

Frederick. — And white, which seems to be no color, 
is all colors combined ! 

Harriet. — What you have told us about colors and light, 
papa, is quite contrary to the ideas I before had about them. 

Mr. P. — There is no branch of science which deludes 
the senses so much as that connected with light and colors. 
The most extraordinary deceptions can be produced by a 
proper disposition of colors, combined with the effect of 
light and shade. The exhibition of the Diorama affords 
a most beautiful example of this ; and by having a contriv- 
ance for diminishing and increasing the light upon partic- 
ular parts of the painting, the effect produced is perfectly 
wonderful. 

Harriet. — Yes, papa ; I remember, when you took us 
there, I could not believe that what we were looking upon 
was only a flat piece of canvass. 



COLORS. 161 

Mr. P. — It is, indeed, difficult to conceive that the 
objects painted on the canvass are not realities. The eye 
is still more easily deceived by colors than by the forms of 
objects. There are some persons who cannot even distin- 
guish one color from another, and make very curious mis- 
takes in consequence. A gentleman, residing in Derby, 
who had this peculiar defect of vision, went to his tailor to 
order a suit of mourning to attend the funeral of a friend, 
and when shown the card of patterns to select his cloth, 
he unfortunately fixed upon a brilliant red. The tailor 
supposed he wanted the dress for a fancy ball, and, without 
hesitation, made up the suit and sent it home, according to 
order. On the morning of the funeral, a friend of the gen- 
tleman's luckily called in to accompany him ; when, to his 
amazement, he beheld the mourner make his appearance 
accoutred from head to foot in his splendid suit of red. 
The effect was rendered the more ludicrous by the serious 
face and unconscious manner of the gentleman ; nor could 
he understand why his friend looked so astonished; and it 
was with difficulty he could be convinced of the impropriety 
of going to the funeral in a dress he had ordered expressly 
for the occasion. 

Harriet. — I never heard of anything so strange. Had 
the gentleman chosen blue, I should not have wondered, 
for I often see people with blue coats and gowns, which 
they imagine to be black. 

Robert. — If persons were to be dressed in perfect 
black, they would be, or rather they ought to be, invisible. 

Mr. P. — So they would be, if there were a back-ground 
equally black behind them. I have seen a very good de- 
ception produced in this way on the stage. The figure of 
a skeleton was painted on a black dress fitting very tight 
to the body of a man, who was enveloped in a long black 
cloak. When he spread out his arms and disclosed him- 
14* 



162 COLORS. 

self, the real outline of his figure was invisible, as there 
was nothing to contrast with it, and there appeared only 
the form of the skeleton, which seemed to be endowed 
with life. 

Harriet. — How very frightful it must have looked ! If 
the skeleton had not been painted on the black dress, I 
suppose you could not have seen any thing of the man 
when he opened the cloak. 

Mr. P. — No ; he would have been perfectly invisible, 
as Robert conceives every one ought to be who is dressed 
in a color that absorbs all the light. 



QUESTIONS 



1. What is the color of light? 

2. Of what is the white light of the sun composed ? 

3. Who discovered the composition of light ? 

4. In what way can the light of the sun be separated into its 
primitive colors ? 

5. What is the solar spectrum ? 

6. Name the colors in order. 

7. What rays are most refrangible ? — What least ? 

8. If the colors of the solar spectrum be reunited by a convex 
lens, what will be their color ? 

9. How can - the same effect be produced by mechanical means ? 

10. Why do the rays of the sun appear of different colors, as they 
are reflected from different objects ? 

11. Why does a rose appear red? — Grass, green? — Gold, 
yellow? &c. 

12. How are the colors of the rainbow produced ? 

13. How can rain-drops, being transparent, reflect light ? 

14. In what way do you account for the brightness of glass 
windows in the sunshine ? — Of rain-drops ? 

15. Explain Figure 14. 

16. Why does not a rainbow always sparkle and glitter ? 



COLORS. 163 

17. What is the secondary rainbow ? 

18. How is it produced ? 

19. How is the halo round the moon produced ? 

20. In what way do you account for the colors of the clouds at 
sunset ? 

21. What is blackness ? — How produced ? 

22. How can we see a black object ? 

23. Why does paper appear white ? 

24. What produces the play of colors in a diamond ? — Cut-glass ? 

25. Why does blue appear green by lamp light ? 



1G4 REFLECTION. 



CONVERSATION XVIII 



REFLECTION. 



Mr. P. — I dare say, Harriet, you have often looked at 
yourself in the glass ; — can you tell us the cause of your 
seeing yourself there ? 

Harriet. — No, indeed, papa ; I never thought of it : 
I should like very much to know. 

Mr. P. — The effect is produced by the reflection of 
the rays of light from the polished surface of the mirror. 

Frederick. — How can the reflection of the rays of 
light produce such an effect? 

Mr. P. — When light strikes against a polished opaque 
surface, it is reflected or sent back, in the same manner 
that all elastic substances, when striking against one 
another, rebound, or are reflected. Thus, when Harriet 
looks at herself in the glass, the rays of light, proceeding 
from all parts of her face, strike upon the mirror, and are 
sent back, or reflected, towards her again ; and her eyes 
receive the reflected rays from the glass, and form them 
into an image of herself. 

Robert. — But how is it that I can now see Harriet in 
the glass, though we are neither of us before it 1 

Mr. P. — That, also, is owing to the reflection of her 
image by the glass. The rays from her face strike against 
the glass at an angle of about 45° from the perpendicular 



REFLECTION. 



165 



of its surface, and they are reflected towards you at 
the same angle on the other side the perpendicular;* 
therefore, you see her image, though she cannot see it 
herself. She is also able to see you, from the same cause. 

* A line is said to be perpendicular to another line, or a plane surface, 
when it inclines neither way. Thus, [see Fig. XVI.] the line C P is perpen- 
dicular to A B. And observe that perpendicular does not mean the same 
thing- as up-and-down; for lines may be perpendicular without being- ver- 
tical. Thus, the line H P is perpendicular to the line R h, and the line h P 
is perpendicular to H r. 

An angle is the opening of two lines which meet in a point, or cross each 
other. Thus, the opening contained between the lines P R and P C, is an 
angle, and it is read R P C or C P R ; the letter which stands at the point 
where the lines meet being put in the middle. The angle C P H expresses 
the opening of the lines C P and P H ; and the angle r P h, that of the lines 
r P and h P. 

Angles are measured by the number of degrees they contain. Every 
circle, whether large or small, is supposed to be divided into 360 parts, 
called degrees ; and, if the angular point be regarded as the centre of a 
circle, a greater or less number of these parts will be contained between the 
lines, as the angle is larger or smaller. Thus, [see Fig. XV.] the angle 

D C B contains more degrees 
Fig. XV. than the smaller angle D C E, 

because a larger portion of the 
circle A D E B F, or a d e b f, 
is contained between the lines 
C D and C B, than between 
the lines C D and C E. In 
measuring an angle it is of no 
consequence whether we use a 
large or a small circle ; the 
number of degrees, and of 
course the opening of the lines, 
will be the same in both, al- 
though the length of the de- 
grees will be different in the two 
circles. 
A right angle contains the quarter of a circle, or 90 degrees. Thus, A C 
D, D C B, B C F, and F C A, are all right angles. 

An oblique angle contains either more or less than 90° j if more, it is 
called an obtuse angle ; if less, an acute angle. Thus, ACE and E C F 
are obtuse angles ; and D C E and EC B are acute angles.— Am. Ed. 





loo REFLECTION. 

Frederick. — Are the rays of light always reflected at 
the same angle at which they strike the reflecting surface? 
Mr. P. — Yes, that is the rule which governs all 
reflections. I shall, perhaps, make the subject more 
intelligible by this drawing. The line a b represents the 
mirror; k is the point where 
°' ' Harriet stood ; r is Robert's 

position : and the dotted line p 
c is the perpendicular drawn 
from the surface of the glass. 
Now, those rays of light from 
Harriet's face, falling on the 
lass in the direction e p, were 
reflected at an equal angle on the 
other side its perpendicular, that 
is, in the direction p r ; and Robert, looking in the same 
direction towards the point p, would see Harriet's image, 
apparently behind the glass at h. The rays from Robert's 
face would, in the same manner, be reflected in the direc- 
tion p ii, and Harriet would see his image behind the glass 
at R. The angle at which the rays of light fall upon the 
plane of the reflector is called the angle of incidence, and 
the angle at which they are reflected is called the angle of 
reflection: these angles are always equal to one another. 
Now that the sun is shining into the room, I can show you 
that the angle of incidence and the angle of reflection are 
equal. You see the ray of the sun's light strikes obliquely 
on the table at an angle of about 30°. Frederick, place 
the small looking-glass flat on the table, and notice where 
the reflection will be. 

Frederick. — There it is, on the ceiling, at the farther 
end of the room. 

Mr. P. — You can perceive, by the motes in the 
sunbeam, the directions of the incident and of the 



REFLECTION. 167 

reflected rays distinctly ; and you will find that the 
angles they make with the surface of the glass are equal. 

Robert. — Will they be so in any direction ? 

Mr. P. — Raise the glass as you like, and you will find 
that the nearer you bring the plane of the glass perpendic- 
ular to the sun's ray, the distance between the reflected 
image and the incident ray will diminish. 

Frederick. — Yes, look ! as I raise the glass, the 
bright spot on the ceiling moves nearer to the window ; 
and now it is entirely lost in the sun's ray. 

Mr. P. — The light from the sun now strikes directly, 
that is, perpendicularly, against the glass, and is reflected 
into itself, as the image of Harriet is when she is admiring 
her own face. When the rays struck the glass obliquely, 
and were reflected to the ceiling, they were in the same 
relative positions to the glass as Robert and Harriet were 
when they saw each other. 

Harriet. — Last evening, I was very much surprised to 
see what I thought was a large fire in the road ; but on 
going to the window to look, I found that it was nothing 
but the fire in the room that I saw. 

Mr. P. — The deception was produced by the rays of 
light from the fire striking against the window, which 
reflected them to you at the other end of the room, in the 
same manner that the rays from Robert's face were 
reflected to you by the mirror. 

Harriet. — Yes, but there was no looking-glass near 
the window. 

Mr. P. — All glass, however transparent it may be, 
reflects a portion of the light that falls upon it ; and when 
the rays strike upon a pane of glass very obliquely, most 
of them are reflected ; as I mentioned to you yesterday 
in explaining the rainbow. 



163 REFLECTION. 

Harriet. — Then why is not the fire reflected by the 
glass in the day-time, as well as in the dark ? 

Mr. P. — It is so ; but the rays of light, from the objects 
outside the window, enter the glass during the day, and, 
being more vivid than the partially reflected rays from the 
fire, obscure them. 

Harriet. — But how was it that the fire appeared to be 
in the road 1 

Mr. P. — The image in a plane mirror, or reflector, 
always appears to be in the direction that the reflected 
rays enter the eye, and to be as much behind the glass as 
the object is before it. It is one of the laws of reflection 
from plane mirrors, that images appear to be formed on an 
ideal line drawn perpendicular from the objects to the 
surface of the reflector. Thus, you observe, in the 
drawing, the point r, where you would see Robert's image, 
is on a line r r, drawn perpendicular to the surface a b. 
When you looked at Robert in the glass, he appeared to 
be behind it ; but you knew from experience that he 
was not ; and the other objects in the room being also 
reflected, tended to destroy the illusion. In the case of 
the -fire, the deception is greater, in consequence of the 
reflected rays from it alone being visible, as the rays from 
the other objects in the room would not be sufficiently 
vivid to be seen. 

Frederick. — Do not you remember, Harriet, when we 
were little children, trying to catch ourselves behind the 
looking-glass? Nothing puzzled me so much as to find 
the image gone when I looked behind. 

Harriet. — Yes, I recollect it very well : and, not long 
ago, I was quite deceived, when at the Bazaar in Soho 
Square, by the reflections in the large mirrors placed 
against the walls : I thought one was an open door leading 



REFLECTION. 169 

into another magnificent room, and T should have walked 
against the glass if I had not been held back. 

Mr. P. — Since the invention of that elegant instrument, 
the kaleidoscope, by Dr. Brewster, the effect of which 
depends upon reiterated reflection, more attention has 
been paid to the reflections from plane mirrors; and 
nothing produces so pleasing and deceptive an appearance 
as a series of them well arranged. 

Frederick. — Are all elastic bodies reflected in the 
same manner as light '? 

Mr. P. — Yes, their reflection is governed by the same 
laws; the reflection being always at the same angle at 
which they strike the reflecting body. In addition to 
those properties of light which we have now considered, 
discoveries have been made respecting its nature, and the 
circumstances attending its contact with and transmission 
through solid bodies, that seem to open a wide field to 
philosophical investigation. The polarization of light, 
the inflection or diffraction of light when passing near 
the edges of bodies, and the effect of the interference of 
light when rays cross each other, though highly interesting 
as subjects of scientific research, have not as yet, 
however, been sufficiently connected with the commonly 
observed phenomena of light to render them fitting 
subjects for our present notice. The nature and proper- 
ties of light that most affect the appearances of objects 
around us are those which I have explained to you, viz. 
its composition of colored rays, its refraction and 
reflection. 

15 



170 REFLECTION. 



QUESTIONS. 

1. Can you tell the cause of your seeing yourself in the glass ? 

2. How does reflection produce the image ? 

3. How can one person see another in a glass, when neither of 
them is before it ? 

4. When is one line said to be perpendicular to another ? 

5. Are perpendicular lines always vertical ? — Show this by 
Figure 16. 

6. What is an angle ? 

7. How are angles read ? — Give some examples. 

8. How are angles measured ? 

9. What is a degree ? 

10. What is a right angle ? — Oblique angle ? — Obtuse angle ? — 
Acute angle ? 

11. Give examples of each. Figure 15. 

12. What angle is made by two lines, which are perpendicular 
to each other ? 

13. How are rays of light reflected which fall perpendicularly 
upon a glass? 

14. How are they reflected if they fall obliquely ? 

15. Give the law by which they are reflected. 

16. Explain Figure 16. 

17. What is the angle of incidence ? 

18. What is the angle of reflection ? 

19. By what experiment are these shown to be equal ? 

20. Where does the image of an object, seen in a plane mirror, 
always appear to be ? 

21 . By what law is all reflected motion governed ? 

22. Why cannot a person see an image of himself, when he looks 
obliquely into water? 

23. When a person sees his image in water, why is it irregular 
and fluctuating? 



VISION. 171 



CONVERSATION XIX 



VISION 



Frederick. — Since you explained to us yesterday, 
father, the reflection of light, I have thought of one 
thing that I do not understand. When the rays of the 
sun, or the light of the fire, are reflected by a looking- 
glass against the wall, I see nothing but a square bright 
light, the shape of the glass ; but if I let the reflection 
come into my face, I see the image of the sun quite 
around, and distinguish the flames of the fire. 

Mr. P. — I am not surprised at your being puzzled by 
the different appearances the rays present, when reflected 
directly on the eye, and when the reflection is thrown on 
the wall : I will endeavor to explain the difficulty. I have 
before told you that the rays of light from the sun, and the 
rays reflected from any visible object, proceed from all parts 
of the object in straight lines ; and that there is no point 
on which the light falls, however small it may be, that 
does not receive rays from every part of the object. Thus, 
there is no point of the wall illumined by the fire, that 
does not receive rays from all parts of it at the same time. 
The light on the wall may, therefore, be considered as com- 
posed of innumerable small images of the fire, which are 
all blended together, and assume an appearance of uniform 
light. When any portion of these mingled rays from the 



l?2 vision. 

fire is reflected, from the surface of a plane mirror, upoH 
the wall, the only effect produced is to increase the light i;i 
that part, by the addition of the quantity of light reflected 
by the mirror; the rays of light being as much mingled 
together as before. But, if you place your eye in a posi- 
tion to catch the reflected rays, all the small fires of which 
the light is composed are brought, by the lenses of the eye, 
to one point, and there form a distinct image of the fire. 

Frederick. — Then the light on the wall is the same 
as that which enters the eye, and the difference in 
appearance is produced by the eye itself? 

Mr. P. — It is : I can show you the manner in which 
this effect is produced, by placing a convex lens,* or 
magnifying glass, between the fire and the wall. But you 
must first close the window-shutters, else the rays of 
light entering from without will overpower the light from 

* A lens is made of glass or some other transparent substance, by which 
the light, that passes through it, may be refracted. 

There are several kinds of lenses, the most important of which are repre- 
sented in the wood-cut annexed. 

Fig. XVII. 
A B CD 




A single convex lens, A, has one side plane and the other convex. 

A double convex lens, B, has both sides eonvex. 

It is the property of convex lenses to bring the rays of light to a focus, 
which will be explained hereafter. 

A single concave lens, E, is plane on one side and concave on the other. 

A double concave lens, D, is concave on both sides. 

Concave lenses disperse the rays of light that pass through them. 

A meniscus, C, is concave on one side and convex on the other. — 
Am. Ed. 



VISION. 



173 



the fire. (Frederick and Robert close the shutters.) 
You perceive, as I hold the glass near the wall, it inter- 
cepts the light ; and as I bring it nearer to its focus, you 
may see a distinct inverted image of the fire on the wall. 

Harriet. — Does the glass collect all the little fires 
together, to form that one % 

Mr. P. — Yes, my dear, all the mingled rays from 
the fire are collected by the glass towards separate 
points, and that is called bringing the rays to. a focus. I 
shall explain how this effect is produced by lenses at 
another time. 

Frederick. — Does the eye bring the rays to a focus in 
the same way as a' lens? 

Mr. P. — Yes; the eye is composed of lenses, of 
different degrees of convexity and hardness. The crys- 
talline lens, which is the principal one, exactly resembles 
in shape a double convex lens, or common magnifying 
glass. By means of these lenses, the rays of light 
proceeding from all visible objects, are converged to a 
focus at the back of the eye.* The same effect may be 



* The human eye is nearly of a spherical form, a section of which is 
represented in Figure XVIII. The external covering of the eye, a a a a, 

is called the sclerotic coat. 
Fig. XVIII. It is a strong membrane, 

and is lined by another, 
cal':ed the choroid, which is 
thinner and more delicate 
The choroid is imbued with 
a black substance, which 
absorbs all the rays that 
are irregularly refracted. 
The sclerotic membrane 
has a projection in front, 
b b, called the cornea, or, 
more commonty, the white 
of the eye. Within the 
choroid lies the retina, h h h h, which is an expansion of the optic nerve, u 

15* 




m 



VISION. 



shown by holding a small lens at the end of the room 
before a white screen, when we shall see the objects 
outside distinctly marked on the screen, though much 
diminished. (Mr. Powell holds the lens at its 
distance from the screen, so as to throw the image upon it.) 

Harriet. — Yes, papa, I see the houses, and the people 
walking in the road ; but they are all topsy-turvy, and the 
men seem to be walking on their heads.* 

Mr. P. — You must understand, that the liouses and 

Immediately back of the cornea is the iris, c c, or the colored j ait of the 
eye ; in the middle of which is the pupil, d. The pupil is a circular opening- 
through which the light is admitted. Behind the pupil is the crystalline 
humor, f, which is a firm, transparent, double convex lens. The space 
between the crystalline lens and the iris r is filled with the aqueous humor. 
e e; and the body of the eye, between this lens and the retina, is occupied 
by the vitreous humor, g g. The light admitted through the pupil is refracted 
by these humors in such a manner, as to form a perfect picture of external 
objects on the retina. — Am. Ed. 

* This effect is exhibited in the following diagram, in which, for the sake 
of simplicity, only the most essential parts of the human eye are represented. 

Fig. XIX. 




In this diagram, a represents the ball of the eye. h the crystalline lens. 
c d an object before the eye, and/e its image on the retina. The diverging 
rays from the top of the object, c, are brought to a focus at e; and those 
from the bottom of the object, d, are brought to a focus at /. In the same 
manner, the rays from every part of the object, c d. are refracted to their 
appropriate place in the image e f, which is inverted on the retina. — 
Am Ed. 



VISION. 175 

other objects in the street give out rays of light during the 
day, as well as the fire ; and the light on the wall of the 
room is produced by an innumerable quantity of the rays 
from the houses, sky, &c. Now, when I place the lens 
between the window and the wall, the surface of the glass 
receives a number of these rays, which are converged by 
it, and formed into the image you see. 

Frederick. — But I suppose the objects are not shown 
upside-down at the back of the eye? 

Mr. P. — Yes, they are indeed. 

Robert. — What ! do you mean, that I see people stand- 
ing on their heads, and trees growing with their roots 
upwards ? 

Mr. P. — No, Robert, / do not mean to say so ; but 
there are few writers who treat on this subject who do not. 

Robert. — Do they seriously say that men see every 
thing in the world upside-down? 

Mr. P. — They are quite serious. 

Frederick. — Then how can they account for my seeing 
Robert and Harriet standing on their feet, if my eyes see 
them with their feet uppermost ? 

Mr. P. — To get over that difficulty, it is said infants 
find, by feeling at different things, that what seems to them 
to be the top, is, in fact, the bottom ; and that so, by ex- 
perience, we get accustomed to refer all objects to a differ- 
ent position from that in which they are seen. 

Harriet. — But do little children, then, really see their 
mammas and nurses with their heels uppermost? How 
very droll it must be ! 

Mr. P. — There is no reason to believe that they do ; 
but as it was taken for granted that the images must be 
seen inverted, because they were so at the back of the eye, 
there was no other means of accounting for their appearing 



176 VISION. 

upright, than by supposing we corrected the inverted 
position by experience when young. 

Frederick. — Then how do you account, father, for our 
seeing things upright, if the images be inverted at the 
back of the eye ? 

Mr. P. — The retina, on which the inverted image is 
formed, is an expansion of the optic nerve ; which nerve 
conveys the sensation on the retina to the brain. No one 
supposes that the retina sees the object ; it is only a screen 
on which the image is formed. It seems, therefore, sur- 
prising, that it should have ever been inferred, from the 
single fact of the image being inverted on this screen, that 
the objects must be seen in an inverted position by the 
mind. 

Robert. — But how can the mind change the position 
of the image on the screen at the back of the eye? 

Mr. P. — The inverted picture formed by the lens on 
the paper screen will solve the difficulty at once. You 
observe that, when you look at the screen in front, the 
houses appear inverted ; but go behind the screen, and 
look down upon the inverted picture. Now, Robert, how 
do the houses seem 1 

Robert. — (Looking down upon the screen, as represented 
in the wood-cut.) Just as they ought to be ; and the people 

are walking very prop- 
Fig. XX. erly on their legs. 

4E& ^ R * **' — ^ he nouses 

^5i™hv st ^ a pp ear t° me to 

JR|B \ \ be inverted; but the 

%p\j' tsA position of the objects 

W^^Z^^'. p| to you is changed by 

W " ~/ ~^rjf ■** •* your seeing them from 

a different point of 
view. In the same manner we may imagine the mind 



vision. 177 

looking down upon the inverted image on the retina, and 
viewing it in an upright position.* As we can form no 
conception as to the manner in which the sensation of sight 
is produced in the brain, and as there is really no more 
difficulty in supposing the objects are seen upright, than 
in the supposition that they are seen inverted, it seems 
absurd to conclude that they are seen in a position the re- 
verse of their real one ; it is gratuitously inventing a 
difficulty, for no object whatever. The inverted theory, 
too, is contrary to that simplicity and harmony of arrange- 
ment, which are always perceptible in the works of Nature ; 
and it ought to have been received with great doubts 
on that account alone. 

Robert. — I do not think I should ever have believed 
the topsy-turvy plan of seeing things upright ; and I am 
very glad, father, that you do not. 

Frederick. — What is the reason that some people are 
short-sighted, and others long-sighted, and are obliged to 
wear spectacles? 

Mr. P. — In the eyes of those persons who are near-sight- 



* For a very ingenious and philosophical solution of the problem, in what 
manner an inverted image produces the sensation of an erect object, the 
reader is referred to Dr. Brewster's Optics, Part III. Chap. xxxv. The 
reasoning, facts, and illustrations, there given, cannot be quoted at length ; 
and they would be out of place in a merely elementary work. The con- 
clusions to which they lead, are, that the lines of visible direction, in which 
the parts of an object are seen, do not depend upon the direction of the rays, 
but are always perpendicular to the retina. And as the interior of the eye 
is nearly a perfect sphere, lines perpendicular to the surface of the retina, 
must all pass through a single point, namely, the centre of its spherical 
surface. This point is called the centre of visible direction, because every 
part of a visible object will be seen in the direction of a line drawn from this 
centre to the visible part. Of course the^lines of visible direction necessarily 
cross each other at the centre of visible direction ; so that those from the 
lower p^rt of the image go to the upper part of the object, and those from 
the upper part of the image go to the lower part of the object.— Am. Ed. 



N 



ITS VISION. 

ed, the lenses of the eye are too convex, and collect the 
rays of light into a focus before they arrive at the retina ; 
the image is, therefore, formed indistinctly upon it. In 
long-sighted people, on the contrary, the lenses of the eye 
are too flat, and do not bring the rays to a focus soon 
enough. To correct these defects of vision, the near-sight- 
ed person must use concave glasses, and the long-sighted 
person must use glasses that are convex. We can see the 
effect of these different glasses on the inverted picture pro- 
duced by the lens on the screen. [Mr. Powell holds the 
lens at its focal distance from the screen.) You see the 
image of the houses is now perfectly distinct, for the screen 
is placed in the focus of the lens. I will remove the glass 
a little farther from it. 

Harriet. — The houses and people seem now to be all 
confused. 

Mr. P. — That is the appearance which objects present 
to those who are near-sighted. The rays are brought to a 
focus before they reach the screen ; as you may perceive 
on placing a piece of paper about half an inch from it. 

.Harriet. — [Holding apiece of paper in the focus of tht 
lens.) Yes, here I see them all again, quite distinctly. 

Mr. P. — I will put a concave glass close to the lens, 
and you will find that it will make the picture on the 
screen again distinct. You perceive it is now clear. 

Robert. — How does the concave glass produce this 
effect 1 

Mr. P. — You must know, that the length of the focus 
of a glass depends upon its convexity or roundness, the 
focus being nearer to the glass when the convexity is the 
greater ; and the flatter the surfaces of the glass are, the 
greater is the distance of the focus. A concave glass, on 
the contrary, instead of bringing the rays of light to a point, 
a r ter passing through it, spreads them farther apart; and 



VISION. 179 

the deeper the hollows of the concave surfaces, the more 
will the rays be separated. Therefore, the effect of the 
concave glass is to separate the rays of light as they are 
converging towards a point, and to prevent them from 
coming tu a focus so near the lens. If the hollows or con- 
cavities of the glass exceed the convexity of the lens, the 
rays would not come to a point at all. 

Frederick. — I suppose, if, instead of the concave glass, 
you were to use another convex one, the rays would come 
to a point sooner than before ? 

Mr. P. — Yes ; and it is upon that principle that people 
who are long-sighted correct the defect. They use convex 
glasses, and by this means the rays of light, that were not 
sufficiently converged by the lenses of the eye, are brought 
to a focus on the retina. As the rays of light, proceeding 
from objects near the eye, diverge more than the rays issuing 
from more distant ones, all objects would have appeared 
confused, except when viewed at a certain distance, if the 
eye had not the power of adapting itself to the different 
circumstances under which it receives the rays of light. 
To accomplish this end, it possesses the faculty — either by 
varying, to a certain extent, the convexity of its lenses, or 
their distance from the retina — of adjusting itself according 
to the divergence of the rays, so as to form the images of 
objects distinctly on the retina, at all distances exceeding 
five or six inches. 

Harriet. — The formation of the eye seems to be as 
complicated as that of a telescope. 

Mr. P. — It is the most astonishing piece of mechanism 
ever beheld, and is most admirably adapted in all its parts 
to the uses for which it is intended. Light, by which all 
things are rendered visible, is inexplicable to us in its nature 
and mode of operation : we know that it is reflected from 
all objects, and that they thus send their borrowed rays to 



ISO VISION. 

the eye — it is, by some unknown means, refracted by lenses 
so adjusted in the ball of the eye as to concentrate the rays 
on the retina to a focus — the retina is spread out as a screen, 
peculiarly well adapted to receive the images that the lenses 
throw upon its sensitive fibres — the optic nerve conveys 
the impression, by some inscrutable process, from the retina 
to the brain, there to excite in the mind, in some manner 
totally beyond our comprehension, the sense of sight. Had 
one link of the chain been wanting, the rest would have 
been useless. The organs of sight could not have been 
brought into action without the existence of light, and light 
itself would have been valueless if not endowed with the 
properties of reflection and refraction. And when we 
admire the wisdom which created the organ, we must not 
overlook the beneficent provision for the continued per- 
formance of its functions, and the. care with which it has 
been secured from injury. The secreted humors of the 
eye preserve its proper form and maintain its transparency ; 
while the bony socket, the shadowing eyebrow, the cur- 
taining lid, and the sensitive eyelash, secure it from vio- 
lence, protect it from injury, and warn it from danger. 



QUESTIONS. 



1. How do the rays from a fire or the sun appear, when reflected 
by a glass upon the wall ? 

2. How do they appear when reflected upon the eye ? 

3. In what way do you explain these different appearances ? 

4. By what experiment is this illustrated ? 

5. What is a lens ? 

6. What is a single convex lens ? — A double convex lens ? 

7. What is the property of convex lenses ? 

8. What is a focus ? 



VISION. 181 

9. What is the form of a single concave lens ? — A double concave 
leai : 

10. How do concave lenses refract rays of light? 

11. "What is a meniscus ? 

I -2. What is the form of the human eye : 

13. Explain Figure 18. 

14. What is the process of seeing: 

15. In what position are external objects represented on the retina 
of the eye ? 

16. How may this be proved ? 

17. If the image on the retina is inverted, how does it happen 
that we see the object erect ? 

18. What is the common opinion on this subject? — What are the 
author's views ? 

19. In what way does he illustrate his opinion? 

20. What defect of the eye causes some persons to be short- 
sighted ? 

21. Why does a short-sighted person bring objects near to his 
eye? 

22. How may this defect be remedied ? 

23. What causes some persons to be long-sighted ? 

24. Why do aged persons hold objects, which they are examining, 
at a great distance from their eyes ? 

25. Do they require convex or concave glasses ? — Why ? 

26. How can the eye see objects distinctly, which are at different 
distances ? 

27. How far must an object be from the eye to be distinctly seen ? 
2~. Why is not the image distinct,, when the object is quite near 

to the eye ? 

29. What is said in the text on the curious mechanism of the eve ? 
16 



182 



VISION. 



CONVERSATION XX 



VISION (continued.) 



Mr. P. — Did any of you ever consider the cause why 
objects appear so much smaller when seen from a distance 
than when viewed near 1 

Frederick. — I have often wondered at their appearing 
so, but I could never find out the cause. 

Mr. P. — The apparent size of objects depends upon the 
angle, which the rays of light, issuing from their extreme 
points, subtend on entering the eye ; and as the rays which 
proceed from the top and bottom of an object, when near, 
enter the eye at a much greater angle than when it is at a 
distance, it appears proportionably so much larger, as this 

drawing will show.* 



Fig. XXI. 



Balls of the same 
size are here sup- 
posed to be placed 
at different distance s 
from the eye at e; 
those at b being 
double the distance 
of the one at a, and 
those at d being 
double the distance 
of those at b. It is evident, that the rays of light from the 

* The angle d e f, under which the four balls are seen, is called the 
angle of vision. — Am. Ed. 




VISION. 183 

two extremes of the four balls ranged in a straight line at 
df, must proceed to the eye in the direction d e and ef, 
forming the angle d e f Now, two of the same balls, 
placed at b, will obstruct those rays, and hide all the four 
balls, and will appear to occupy as much space as the four. 
Again, the single ball at a will conceal all the others, and 
will, consequently, seem to occupy as much space as the four 
at df, and as the two at b; therefore, the same ball, placed 
at a, will appear four times larger in diameter than when 
placed in the line d f which is four times the distance 
from the eye.* 

Frederick. — And if the ball were removed farther off, 
I suppose it would appear less and less, in the same pro- 
portion, till it was out of sight. 

Mr. P. — Yes, it would. 

Robert. — I should suppose the ball might be seen, how- 
ever far off, if the day were clear, and there was nothing 
to hide it. 

Mr. P. — No, Robert, it could not ; for, at a certain 
distance, the angle subtended by the extreme points of the 
ball to the eye, would be so minute that the retina would 
not be sensible to the impression the light from it would 
produce. The human eye cannot perceive any object, in 
an ordinary light, that is smaller than the one hundredth 
part of an inch, when held at a distance of six inches from 
it. If an object be smaller than that, or be at such a 
distance that the rays from its extreme points do not 
occupy, when at a distance of six inches from the eye, a 
space equal to the one hundredth part of an inch, it will be 
invisible. The figure of a man is so much diminished by 
distance, as to be invisible to the naked eye four miles off; 

* In this figure we see but one diameter of the balls. If for the diagram 
we substitute real balls, the one ball at a will conceal four balls at b, aud 
sixteen at d.-— Am. Ed. 



134 VISION. 

for then the rays from his head and feet are so close to- 
gether, as to be covered by the point of a pin. 

Robert. — But if the same thing placed near the eye 
appears as tall as a much larger one at a distance, how is 
it that we do not mistake them to be really of the same size ? 

Mr. P. — It is owing principally to the diminution of 
the quantity of light as objects recede. I suppose I need 
not remind you that the light from an object decreases four 
times in- quantity when the distance is increased only 
twice,* and that it becomes less and less, in the same 
proportion, the farther it recedes. Now we know, by expe- 
rience, that the sizes of objects diminish as they are 
removed from the eye; therefore, when we see a person, 
with whom we have been talking, walk away, we do not 
imagine that he grows less as he departs, but attribute his 
apparent decrease in size to his being at a greater distance 
than before. We can compare him also with other objects 
whose size we are acquainted with ; and as they are dimin- 
ished by distance equally with himself, he still appears to 
be of the same relative size, however much the apparent 
magnitude of his figure is lessened. And again, as his 
person recedes from view, the light from it decreases in 
fourfold proportion to the distance, and we are therefore 
accustomed, by connecting together the two circumstances 
of size and brightness, to conceive what impression a man 
of his size, at such a distance, must produce on the retina. 
If we saw the same person approaching at another time, 
when there were no objects to compare him with, we 
should thus know, from the impression he produced, that 
he was really taller than a child close at hand, who might 
seem to be much greater than his image on the eye. It 
must be admitted, however, that we are frequently much 
deceived in the size and distance of objects, when there 

* See Conversation on Light. 



vision. 185 

are no other known objects near them by which to form a 
comparison. 

Frederick. — What you have told us about judging of 
the size of objects by comparison, puts me in mind of the 
curious deception we witnessed at the exhibition of the 
Fantoccini the other evening. When the exhibitor him- 
self came on the stage, I thought he was an immense 
giant; for, after looking a length of time at the small 
figures, and the scenery painted to correspond with them, 
he seemed, in comparison, to be prodigiously large. 

Mr. P. — It is a very good illustration of the subject, 
Frederick. Similar deceptions may be produced at any 
time by contrasts different from those we have been ac- 
customed to see ; and in the dusk of the evening, when 
our principal guidance as to the size of objects depends 
upon the angle at which they are seen, we frequently mis- 
take smaller and nearer things for larger and more distant 
objects. 

The art of perspective depends upon the application of 
the principle that objects diminish in apparent size as they 
are more remote. The size of objects represented in a 
painting, is drawn in proportion to the distances at which 
they are supposed to be placed; and the effect is 
materially aided by the coloring and shading, which, for 
distant objects, is made faint and indistinct, increasing in 
brightness as the objects approach the foreground, until, 
in some cases, they appear to stand out of the canvass. 

Harriet. — Is the apparently slow motion of carriages 
at a distance, owing to their being seen so small? 

Mr. P. — Yes, my dear, it is ; for the length of the road 
is diminished by distance as well as the size of the car- 
riage ; so that the space of a mile, in a road seen far off, 
may be comprised within the diameter of a common ring, 
held at six inches from the eye. A carriage moving along 
16* 



186 vision. 

a road, viewed from such a distance, at the rate of twelve 
miles an hour, would not cross the diameter of the ring in 
less than five minutes, and you would be scarcely able to 
see that it was in motion ; but if the carriage were to pass 
within twenty yards at the same speed, while you were 
looking through the ring, it would cross the diameter in a 
second of time, and appear to be moving with great 
velocity. The motions of the earth and of the planets are 
not visible to the naked eye, in consequence of the great 
distance of the heavenly bodies. On looking at the moon, 
however, with a powerful telescope, the motion of the earth 
round its axis is very perceptible, for the moon is then seen 
moving off the field of view as the earth turns round. 

Harriet. — I wish you would let us look through your 
telescope, that we might see it ; for I should like very 
much to see the world really turning round. 

Frederick. — Can you tell me, father, what it is that 
makes a lighted stick, when moved about quickly in the 
dark, appear as a line of light '? 

Mr. P. — It is occasioned by the property which the retina 
possesses of retaining the impressions of objects a short 
time after the objects are removed. Impressions remain 
on the retina about the sixth part of a second : therefore, 
the removal of any object, that returns to the same point 
within that time, is not perceived, as the impression is re- 
newed before the absence of the object has been discover- 
ed ; and its track, during th° sixth part of a second, appears 
as a line of light. An ingenious toy has been constructed 
to illustrate this effect of the duration of impressions on the 
retina. A drawing is made on each side of a card — a bird 
on one side, and a cage on the other, for instance — and, by 
turning the card quickly round, both figures appear togeth- 
er, and the bird is seen in the cage. This toy is caoable 
of producing many pleasing and ludicrous effects. 



VISION. 187 

Robert. — You have not told us, father, what is the use 
of two eyes, and how it is, that with two eyes we do not 
see double. 

Mr. P. — The use of two eyes is to increase the light ; 
for, with one, objects appear only half as bright as when 
seen with two. Single vision with two eyes is produced 
by the axes of them being turned to the same object, and 
thereby occasioning the same impressions to be excited on 
botli retinas. These impressions are conveyed by the optic 
nerves to the brain, to produce the sense of sight in the 
mind. Persons who squint must see different objects with 
each eye, though they acquire the habit of attending only 
to one at a time ; and those animals which have their eyes 
placed in opposite parts of their heads, must also receive 
different impressions from each eye. 

Harriet. — Does the colored part of the eye let any light 
through to the screen at the back 1 

Mr. P. — No ; the rays are admitted only through that 
dark spot, in the centre of the eye, called the pupil. 

Harriet. — That part is much larger in the cat's eye 
than it is in ours, and becomes larger and smaller as she 
likes ; what is the reason of that, papa ? Can she see more 
than we can 1 

Mr. P. — No, my dear, she cannot see any more objects ; 
but we may suppose that to her they appear brighter than 
they do to us, as she can admit so many more rays of light 
when the pupils of her eyes are distended than can be 
admitted into ours. 

Harriet. — Sometimes the black part of her eye appears 
not thicker than a hair, and at other times it is quite round. 

Mr. P. — When the light is very powerful, the cat con- 
tracts her eyes, so as to admit but a small quantity ; and 
when the light decreases, she enlarges the pupils, and is 
enabled to admit perhaps ten times more light than can 



188 VISION. 

enter our eyes. She is thus able to see objects when there 
is not sufficient light for us to distinguish them. 

Harriet. — Then it is true that cats can see in the dark ! 
I wish the pupils of our eyes would open and close in the 
same way. 

Mr P. — They do open and close, Harriet, though not 
to such an extent as those of the cat. When we are in a 
strong light, the pupil contracts, because too much light 
occasions pain to the delicate nerves of the eye. As the 
light decreases, the pupil expands, and you may notice a 
very perceptible difference in the size of the pupils when 
exposed to a bright light and when coming out of a dark 
room. Look, Harriet, at Robert's eyes, now that he is in 
the glare of the sun, and we will get him to go into the dark 
closet for a few minutes, that you may see whether any 
change will take place in the size of the pupils. 

Harriet. — The black spot seems very small at present. 
— Now, Robert, go into the dark, and let us see if you have 
cat's eyes, when you come back. 

(Robert goes into the closet, and when he has been shut 
up there a short time, Mr. Powell tells Mm to come back 
again. Harriet pulls him towards the window, into which 
the sun is shining, to look at his eyes, zohen Robert cover* 
them with his hands, and utters an exclamation of pain.) 

Harriet. — Do not hold your hands to your eyes, Robert ; 
I cannot see them. 

Robert. — The light hurts them so, that I can scarcely 
bear it. (Robert retires a little distance from the window, 
and then withdraws his hands from his face.) 

Harriet. — Yes, now I see ; the black part has, indeed, 
spread itself out to twice the size it was before you went 
into the dark. But what made you cry out, Robert ? Cats 
do not scream when they swell the pupils of their eyes ! 

Robert. — I do not know how cats manage, but I know, 



VISION. 189 

Harriet, you would have cried out if your eyes pained you 
as mine did me. 

Mr. P. — The pupils of Robert's eyes having enlarged 
themselves in the dark, they admitted more rays, when he 
returned to the light, than the retina could bear without 
pain, because the human eye has not the power of imme- 
diately contracting itself, like that of a cat. If cats could 
not contract the pupils of their eyes more rapidly than we 
can, they would suffer intense pain on coming from the 
dark, with their pupils fully enlarged, into the light of 
the sun. 

Frederick. — I have observed that when I go out of 
doors at night, from a lighted room, I cannot see any thing 
at first, though, after I have been out a short time, I can 
see tolerably well. I suppose this is owin g to the pupil of 
the eye being small at first, and afterwards enlarging and 
admitting more rays. 

Mr. P. — It is so ; and you may therefore conceive that 
if the pupils of your eyes were capable of being more en- 
larged, you would see objects on a dark night still more 
distinctly. You may also understand, from that circum- 
stance, how it would be possible for eyes more sensitive 
than ours to see objects, when all is perfect darkness to us, 
as I mentioned to you in our conversation on light. 



QUESTIONS. 






1. Upon what does the apparent size of objects depend? 

2. What do you understand by the angle of vision ? 

3. By what law does the angle of vision diminish, as the object is 
removed farther from the eye ? 

4. Explain this law by means of Figure 21. 



190 VISION. 

5. Why can we not see very distant objects ? 

6. What is the smallest angle under which an object can be seen ? 

7. How far can a man be seen ? — Why no farther ? 

8. Why does not a large object at a distance appear to be smaller 
than it really is ? 

9. Why do we often mistake as to the size of things in the 
evening? 

10. Can you account for the fact, that objects generally appear 
larger in a fog than when it is clear ? 

11. On what principle is the art of perspective founded ? 

12. How are distant objects represented in a painting ? 

13. How do you explain the apparently slow motion of carriages 
at a distance ? 

14. Why can we not see the planets move ? 

15. What makes a lighted stick, when moved about quickly in 
the dark, appear like a circle of fire ? 

16. How long do impressions remain on the retina of the eye ? 

17. What is the use of two eyes ? 

18. What is the reason that, with two eyes, we do not see objects 
double ? 

19. Does the colored part of the eye let any light pass through it ? 

20. What is said of the eyes of a cat? 

21. What effect has a strong light upon the pupils of our eves ? — 
What effect has darkness ? 

22. Why is light painful after we have been in the dark ? 



MAGNIFYING GLASSES. 191 



CONVERSATION XXI 



MAGNIFYING GLASSES. 



Robert. — You said, father, the other morning, that the 
power of magnifying glasses depends upon the refraction 
of light : I wish you would explain how refraction can make 
things seem larger than they are. 

Mr. P. — The effect of magnifying glasses, or convex 
lenses, as they are generally termed, is produced by their 
being so formed as to converge the rays of light, by re- 
fraction, on passing through the glass. You must not 
suppose, however, that objects viewed through such a glass 
appear larger than they are ; the apparent increase in their 
size is owing to our being able to look at them nearer 
through a convex lens than we can do with the naked 
eye. 

Robert. — Surely, father, you do not mean to say that a 
fly is really as large as it seems to be when I am looking 
at it through a good magnifying glass ? 

Mr. P. — The same object appears large or small ac- 
cording to the distance at which it is seen, as I explained 
to you in our Conversation on Vision ; and a small object, 
seen near, appears larger than one many times its size at 
a distance. Thus, an object the length of an inch, at the 
distance of a foot from the eye, will appear as large as — 
will conceal from view — an object one hundred times its 



19*2 MAGNIFYING GLASSES. 

size, placed at a distance of one hundred feet ; and the 
shadow of a pea, when held near the eye, will thus cover 
the face of the full moon. When a distant object — the 
figure of a man, for instance — approaches us, it is in- 
creasing in apparent magnitude every step the man ad- 
vances ; and when he comes within one foot, his face 
appears, probably, several hundred times larger than 
it did when first seen. Yet we do not say, in this case, 
that his face is magnified, because, as our eyes can see 
objects when only five or six inches distant, every thing 
we are accustomed to look at, within that distance, ap- 
pears to be only of what we call its natural size. Now, 
when, by means of a lens, we are enabled to look at objects 
more closely than we can with the naked eye — suppose at 
three inches instead of six — the apparent increase of size 
we consider unnatural, and we say the objects are mag- 
nified. To a person who is very short-sighted, and accus- 
tomed to view objects as near as three inches, an object 
seen at that distance he would consider only of its natural 
size. 

Robert. — Then how can we tell what is the real 
natural size of things? 

Mr. P. — The meaning of the term " natural size " is 
merely, that objects seen at a certain distance appear of 
the same size as we have been accustomed to see such 
objects at such a distance. To a fly, we must suppose 
that objects appear as large as they do to us when viewed 
through our most powerful magnifiers, yet to it they are 
then only of their natural size. If we were accustomed to 
view all things as closely as a fly must do, they would 
appear to be equally increased in size as when viewed 
through a microscope, yet we should not then consider 
them to be magnified, any more than we now think a man 
is magnified by coming nearer to us. 



MAGNIFYING GLASSES. 



193 



Frederick. — How is it, father, that a magnifying glass 
enables us to look at things so closely ? 

Mr. P. — It is by making the diverging rays issuing 
from a near object parallel, by their refraction on passing 
through it. The image of the object is thereby formed 
distinctly on the retina at the back of the eye, which it 
cannot be when the rays diverge much on entering it. I 
will, however, explain the manner in which the rays of 
light are refracted, on entering and on passing out of a 
lens, to produce this effect The kind of lenses usually 
employed are double convex lenses ; that is, glasses rounded 
out on each side, like two watch-glasses joined together ; 
but the explanation will be more clear if we consider, at 
first, the refraction in glasses that are rounded on one 
side only and flat on the other. These glasses are called 
plano-convex lenses. 

Harriet. — Then those must be the shape of a single 
watch-glass, laid flat upon the table. 

Mr. P. — Exactly so. Suppose a e [Fig. XXII.] to be 
such a glass seen edge-wise ; the rounded part of which, 

a d e, forms a portion 
Fig. XXII. of the dotted circle 

whose centre is c. 
The lines g h i repre- 
sent parallel rays of 
light falling upon the 
convex surface of the 
glass. On entering 
the glass, these rays 
will be refracted near- 
er to the perpendicu- 
lar of the surface, as 
before explained to you.* The line drawn from the centre 

* Conversation XVI. 

17 




194 MAGNIFYING GLASSES. 

of a circle to its circumference is, as you are aware, per- 
pendicular to that point through which it is drawn ; there- 
fore the line c I will be perpendicular to the surface of the 
glass at the point where the ray g enters. 

FREnERiCK. — I see you have made the ray g go through 
the glass to m in the same direction in which it enters ; 
but, of course, it must be refracted towards the perpen- 
dicular. 

Mr. P. — Yes, just so. The dotted line is continued 
merely to show the amount of the refraction ; for as soon 
as the ray enters the glass, it is bent one third nearer to the 
perpendicular, in the direction np. On coming out of the 
glass into the air, it is again refracted, and in the same 
degree, from the perpendicular of the surface from which 
it issues. The perpendicular to this second surface, a e, 
is the line m, and the refraction of one third from this 
perpendicular will take the ray of light in the direction 
n f to the other extremity of the diameter of the dotted 
circle. The ray h, as it enters and passes out of the glass 
perpendicularly to its two surfaces, will not undergo any 
refraction, and will arrive in a straight line at the point f 
The other ray, i, will be subject to the same refraction as 
the ray g was, and will, in a similar manner, be refracted 
to f, where the three rays will meet. Any other parallel 
ray entering the glass will, by undergoing the same refrac- 
tion, arrive at f where all the rays will concentrate in a 
point, and that point is called the focus, as all the rays of 
light and heat from the sun are centred there in a burn- 
ing glass. 

Frederick.- — Then the focus of such a glass will be 
just the length of the diameter of the circle that forms the 
convex side? 

Mr. P. — Yes, Frederick, that is the case when the re- 
fracting power of the medium is one third, as in common 



MAGNIFYING GLASSES. 



195 




glass ; but when the refraction is greater than one third, 
the rays will come to a point nearer the lens, and the con- 
trary when the refractive power is less. 

In a double convex lens made of glass, the focus will 
be in the centre of the circle, as I will now show you. 

Let n p [Fig. XXIII.] be the 
Fig. XXIII. direction in which a ray of 

light will be refracted on en- 
fering the first convex sur- 
face of the lens u e, as in 
the last figure. On coming 
out of the glass into the air, 
the ray will be refracted 
one third from the perpen- 
dicular of the convex surface. 
The perpendicular to this 
surface, at the point n, is the 
line x y, drawn from the 
centre, x, of the circle that forms the second convex side 
of the lens. You see, therefore, that the directions of the 
ray of light, and of the perpendicular from which it is to 
be refracted, differ much more than they did in the other 
lens ; in which the direction of the perpendicular from its 
fiat surface was n m. As the degree of refraction is in 
proportion to the difference between the direction of the 
ray and that of the perpendicular, you must see that the 
more these differ, the more will the ray be bent from its 
former course. 

Harriet. — I thought the degree of refraction had been 
always the same. 

Mr. P. — It always bears the same proportion to the 
perpendicular of the refracting surface ; but the greater the 
difference between that perpendicular and the direction of 
the incident ray, the greater will be the amount of that 



196 MAGNIFYING GLASSES. 

proportion ; in the same way that the third of nine inches 
is greater than the third of three. For instance, in the 
single convex lens, the difference between the direction of 
the ray and the perpendicular was only from m to p, and 
the proportionate refraction brought the ray to f. In the 
double convex lens, however, the difference is more than 
double, and the refraction, to bear the same proportion to 
it, must consequently be more than twice as great ; and 
the ray, that before was refracted to/, will be bent towards 
the centre c. 

Frederick. — Is the focus of all such lenses, then, in the 
centre of the circle 1 

Mr. P. — Yes, after allowing for the thickness of the 
glass, which, in some lenses, bears a considerable propor- 
tion to their focal distance. I have been thus particular in 
tracing the refraction of the rays of light through a lens till 
they are brought to a focus, as this is seldom attempted to 
be explained in a manner intelligible to young persons. I 
hope I have succeeded in making you understand the sub- 
ject : but the most satisfactory mode of impressing it upon 
your minds is, for you to make a drawing of each kind of 
lens, on a scale large enough to enable you to measure the 
degrees of refraction accurately. 

Robert. — I will make such a drawing this afternoon, and 
then I shall see whether all the parallel rays will be refracted 
by the same rule into the centre. 

Mr. P. — You must bear in mind in your drawing, that 
the degree of the refraction, both to and from the perpen- 
dicular, is one third of the whole after the amount of the 
refraction is added ; and that, therefore, it is one half of 
the length of the line drawn between the direction of the 
ray and the perpendicular. Thus, you perceive, in the last 
drawing, p c, the amount of the refraction is one third of 
the whole line c y, but it is also the half of p y. 



MAGNIFYING GLASSES. 197 

Robert. — I am glad you have mentioned that, father, or 
I should have made a mistake, and have added only one 
third the difference between the ray and the perpen- 
dicular. 



QUESTIONS. 



1. Upon what does the power of magnifying glasses depend ? 

2. Do they make objects appear larger than they really are ? 

3. How do you explain the apparent increase in the size of 
objects, seen through a good magnifier? 

4. What do you understand by the natural size of an object? 

5. How do magnifying glasses enable us to examine objects near 
the eye ? 

6. What is the usual form of magnifying glasses ? 

7. What kind of a glass is represented by Figure 22 ? 

8. How is light refracted by a single convex lens ? — Explain 
Figure 22. 

9. What is the focal distance of a single convex lens ? 

10. Explain Figure 23. 

11. What is the focal distance of a double convex lens? 

12. How much is light refracted in entering and passing from a 
convex lens of glass ? 

13. What allowance must be made, when we calculate the focal 
distance of such a lens ? 

17* 



198 IMAGES OF OBJECTS 



CONVERSATION XXII 



IMAGES OF OBJECTS. 



Mr. P. — Well, Robert, did you make your drawing last 
night, and were you satisfied that all parallel rays are 
brought to a focus at the same point 1 

Robert. — Yes ; I found that the parallel rays entering 
near the edge of the glass are refracted more than those 
near the middle, owing to their being farther from the per- 
pendiculars of the two convex surfaces, which makes up 
for the difference in their distances from the centre. It 
was that which puzzled me at first. 

Mr. P. — Having now traced the rays of light to a focus. 
I will proceed to explain some of the phenomena which 
this concentration of the rays of light produces. The 
burning glass is the most obvious illustration of this effect 
of convex lenses. 

Harriet. — Are burning glasses nothing but glasses 
rounded out on each side 1 

Mr. P. — They are nothing but convex lenses, in which 
all the light from the sun that falls upon their surfaces is 
brought to a point, by refraction, in the manner described, 
and forms in the focus a small image of the sun. As most 
of the light and heat, spread over the surface of the burn- 
ing glass, is concentrated in that small round spot, that 
spot must contain nearly the same quantity of light and 
heat that was before diffused over the glass, and its inten- 



IMAGES OF OBJECTS. 199 

is proportion ably increased. The quantity of heat 
from the sun is the same ; but the intensity of it, on one 
point, is increased by taking heat from other parts. Thus, 
the space round the bright image is always dark in conse- 
quence of the rays of light, that would otherwise have fallen 
there, being directed to the focus of the lens. 

Frederick. — Then, I suppose, when the surface of the 
glass is one hundred times larger than the bright spot, the 
heat on that point would be one hundred times greater than 
the common heat of the sun. 

Mr. P. — It would be so if all the light passed through 
the glass ; but a portion of it is always obstructed even by 
the most transparent substances. Burning glasses have 
been constructed sufficiently powerful to melt plates of iron, 
and to convert slates into glass, in a few minutes. Such 
glasses, however, scarcely act upon transparent bodies, and 
the sun's rays may be concentrated by them on a glass of 
clear water without any effect, as the rays pass through it ; 
but, if the water be discolored, it will speedily boil. The 
reflected rays of light, proceeding from all objects, are con- 
centrated in the same manner as the direct rays from the 
sun ; as I have shown you frequently in our conversations 
on vision, by making all the objects outside the window 
visible by collecting their rays in the focus of a convex lens ; 
and I will now repeat the experiment. {Mr. Powell 
holds a convex lens between a white screen and the window, at 
its focal distance from the screen, so as to form a distinct 
inverted image of the objects in the road.) 

Robert. — But the rays of light are not brought to a 
point now, as they are by a burning glass, and the picture 
on the screen is larger than the glass itself. 

Mr. P. — I will endeavor to explain why the images 
should appear larger than the glass, though the rays of 



200 



IMAGES OF OBJECTS. 



light are brought to the focus. Suppose the lines a c, a d, 
a I [Fig- XXIV.] to represent rays of light proceeding from 

Ficr XXIV. 




the point of the arrow a r, and falling upon the surface 
of the double convex lens c d I. 

Harriet. — Are the rays of light from so small a thing 
as the point of an arrow spread all over the surface of 
the glass. 

Mr. P. — Yes, my dear ; every point of an object, as I 
have previously informed you, sends out rays of light; and 
we could not see the whole of an arrow unless rays pro- 
ceeded from every part of it to the eye. The rays from 
the point of the arrow, therefore, in passing through the 
glass, are refracted in the directions c i, d i, and / 1, form- 
ing at i — where we will suppose them to be brought to a 
focus — the image of the point a. I have here only drawn 
three of the rays from the point a, to avoid confusion ; but 
you must understand that rays proceed from the same point 
to all parts of the surface of the glass, and are reflected to 
i, where they contribute to form the image. In the same 
manner, the rays from the other end of the arrow are con- 
centrated at m, and those from all the intermediate parts 
are brought to their several points between m and i, form- 
ing there a complete inverted image of the arrow. 

Frederick. — So trrat all the separate rays are brought 



IMAGES OF OBJECTS. 201 

to separate points ; and, as the different rays come to the 
glass in different directions, they are brought to points in 
different parts of the screen. 

Mr. P. — Exactly so. 

Harriet. — But what makes the houses and trees and 
men appear to be upside down ? 

Mr. P. — You observe, in the figure, that the central 
rays of every pencil of rays, from the top and bottom of the 
arrow, proceed in straight lines through the centre of the 
glass, and continue in the same direction in passing out of 
it. Thus, the rays a d and r d cross one another in the 
middle of the lens, and the ray from the top of the arrow 
proceeds to i, and that from the bottom, to m; and as all 
the other rays converge round the central ones, the image 
must be inverted. You may convince yourself that the 
image would be formed in this manner, by accurately 
measuring the refractions of the different pencils of rays 
in a drawing on a large scale, such as Robert made 
yesterday. 

Robert. — I see you have made the image larger than 
the object ; — how can that be, father 1 

Mr. P. — The size of the image depends upon the dis- 
tance at which it is formed from the lens. I will show 
you that this is the case by first using the same lens I have 
just employed, and then substituting one of a longer focus. 
(Mr. Powell forms images of the objects outside the window 
upon the screen with the tioo lenses, as described.) 

Harriet. — Every thing appears much larger with the 
second glass, but not so bright, nor so distinct, as before ; — 
what makes the difference ? 

Mr. P. — This figure [Fig. XXV.] will make the cause 
quite evident. The arrow, a r, is supposed to be sending 
out rays of light to the lens, I n. The central rays of each 
pencil will cross in the centre of the lens, as I have already 



202 



IMAGES OF OBJECTS. 



stated, and, after passing through, will diverge towards 
b c. If the rays be brought to a focus at b c, a small image 
will be formed there, equal in length to the distance be- 

Fig. XXV. 




tween the diverging lines proceeding from the two extrem- 
ities of the object. If the image be formed at twice the 
distance, as at d e, it will be twice the length ; and, at four 
times the distance, it will be four times the length, as at 
/ g. But, as the same quantity of light passes from the 
object through the lens in each case, it must be diffused, 
when the image is large, over a much greater surface ; and, 
therefore, as the image increases in size, it diminishes in 
brightness. 

Frederick. — The image at d e seems to be the same 
size as the object. 

Mr. P. — Yes, because it is at the same distance from 
the lens. . I need not, perhaps, tell you, that, when two 
straight lines cross one another, their opposite angles 
are equal, and that, at equal distances from the point 
at which they cross, the lines will be equally distant. 
As the rays from the top and the bottom of an object 
cross in the middle of the lens, the length of the image 
formed behind it will depend upon the distance from 
the lens at which it is formed. Thus the object a r, in 
this figure, is twice as distant from the glass as the first 
image b c, and it is twice the length. The image d e and 



IMAGES OF OBJECTS. 203 

the object are equal in size, and are at equal distances 
from the glass ; while the image at f g, which is twice 
the diameter of the object, is at twice its distance from 
the lens. 

Frederick.— Does the same rule answer for all the 
objects seen out of the window. 

Mr. P. — Yes, it will apply even to the sun himself; for 
the small image formed in the focus of a burning glass 
bears the same proportion to the size of the sun, as the 
distance of the focus from the glass bears to the distance 
of the glass from the sun. 

Harriet. — Then, if we could only get a screen large 
enough and far enough off, and a glass of proper focus, we 
might make the sun's image as large as himself. 

Frederick. — And, even on a small scale, we might, I 
suppose, if we knew the distance of the sun, measure his 
size by measuring the image and its distance from the 
glass. 

Mr. P. — Yes, Frederick, or by knowing the size of the 
sun, you might, in the same manner, estimate his distance 
from the earth. Measurements might be taken, in this 
way, of inaccessible heights, or of other objects, by first 
ascertaining their exact distance, and then measuring the 
size of their images and the focal length of the lens. For 
instance, if the image of a tree, 400 yards off, were formed 
at a distance of one yard from a lens, and measured 
exactly one inch, then we should know that the height of 
the tree was 400 inches, that is, 33 feet 4 inches, 

Frederick. — If we know the distance of the focus, 
would not that be enough, without measuring the distance 
of the image from the glass ? 

Mr. P. — No, it would not ; for when an object is near, 
and the rays diverge from it on entering the lens, they are 
not brought to a focus so soon ; and the nearer the object 



204 IMAGES OF OBJECTS. 

is brought to the lens, the more distant and the larger will 
be its image. When the object arrives at the focal 
distance, the rays from it will be so divergent, that on 
passing through the lens, they will be rendered parallel, 
and not form any image. If the object be brought still 
nearer, the rays will continue to diverge on passing through 
the glass, but their divergence will not be so great as it 
was before. 

Harriet. — I do not yet understand, papa, why we are 
able to look at things nearer with magnifying glasses. 

Mr. P. — I have just now mentioned that, as parallel 
rays are converged to a focus on passing through a convex 
lens, so, on the contrary, rays of light diverging from the 
focus to the lens, are made parallel by their refraction 
on passing through the glass ; therefore, an object placed 
in the focus of such a lens may be seen distinctly through 
it by the eye, however near the focus of the glass may be. 
If, then, a person who cannot see an object clearly when 
placed nearer to his eye than eight inches, is able, by a 
convex lens of one inch focus, to see the same object eight 
times nearer than before, it will be magnified eight times 
in diameter. If, again, he use a lens whose focal distance 
is not more than the twentieth part of an inch, it will make 
objects appear to be 160 times longer, and their surface 
25,600 times larger. 

Harriet. — And will it magnify so many times merely 
by your being able to look at things so much nearer ? 

Mr. P. — Yes ; that is the only way by which the aston- 
ishing effects of magnifying glasses are produced ; for the 
nearer you can view an object, the larger it will appear, as 
I mentioned in our conversation on vision. 



IMAGES OF OBJECTS. 205 



QUESTIONS. 

1. How does it happen that all parallel rays, which fall upon a 
convex lens, are brought to the same focus ? 

2. What are burning glasses ? 

3. What is said of the heat concentrated at the focus of a burn- 
ing glass ? 

4. Why is the space about the focus dark ? 

5. Why cannot clear water, and other transparent substances, be 
heated by a burning glass ? 

6. Can rays, reflected by different objects, be brought to a focus, 
like the direct rays of the sun ? 

7. By what experiment may this be proved ? 

8. What fact is illustrated by Figure 24 ? — Explain the Figure. 

9. Why are the images of objects, represented in this way, 
inverted ? 

10. Upon what circumstance does the size of the images 
depend ? 

11. When the image is large, why is it faint? — Explain 
Figure 25. 

12. What proportion does the small image of the sun, formed in 
the focus of a burning glass, bear to the size of the sun itself? 

13. Why will a magnifying glass give no image of an object 
placed at its focal distance ? 

14. What effect has a convex lens upon parallel rays ? — Con- 
vergent rays ? — Divergent rays ? 

15. How much is an object magnified, when viewed through a 
lens of one inch focus ? — When the focal distance is one twentieth 
of an inch ? 

16. What is the reason of such an apparent increase of size ? 

18 



206 OPTICAL INSTRUMENTS, 



CONVERSATION XXIII 



OPTICAL INSTRUMENTS. 



Mr. P. — We will now apply the knowledge you have 
gained respecting lenses, and the formation of the images 
of objects by them, to explain the cause of the magnifying 
powers of telescopes and other optical instruments. 

Harriet. — I have often wondered, when looking through 
your telescope, how distant things could be brought so near 
as to make me think I could almost touch them ; and I 
shall be quite glad to know how it is. 

Mr. P. — Refracting telescopes, which are those most 
commonly used, consist of a convex lens placed at the large 
end of the tube, and called the object glass. The focus of 
this glass is, in general, nearly as long as the tube. Its use 
is to form an image of the objects looked at ; and the longer 
the focus, the larger the image will be, as I mentioned to 
you yesterday. Suppose an object thirty feet high were 
seen at a distance of 360 yards, and the image of it 
were brought to a focus three feet from the object glass, 
such an image would be 360 times less than the object ; 
that is, one inch long. This image, viewed at a dis- 
tance of one yard, would appear the same size as the 
object does when viewed by the eye from the end of the 
telescope, as will appear from this figure. Suppose c d 
[Fig. XXVI.] to be the object, I n the lens, and i m the 
image. I will now draw a b at the same distance be- 



OPTICAL INSTRUMENTS. "207 

fore the lens as the image is behind it ; and (because it 
is drawn between the converging rays from the two ex- 
Fig. XXVL 



m 



iremes of the object, which cross each other ate,) it will be 
also of the same size as the image. It is evident, there- 
fore, that to an eye situated at the point e, a b will just 
cover the object, and will appear as large to the eye. 

Harriet. — But if the object glass make a thing appear 
only of the same size as when looked at by the eye, what 
good does it do? 

Mr. P. — You forget, Harriet, that we have as yet sup- 
posed the image to be viewed not nearer than at the dis- 
tance of one yard, that is, thirty-six inches, from the eye; 
but most persons would be able to look at it six times 
nearer than that, and it would then appear to be six times 
longer than when seen by the naked eye. Again, by using 
a second convex lens, called an eye glass, of one inch focus, 
we can look at the image when only one inch from the eye ; 
and it would then appear thirty-six times larger in diameter 
than the object, and thirty-six times thirty-six, that is, 1296 
times larger in surface. 

Frederick. — Is it in this manner, then, that telescopes 
are made to magnify 1 

Mr. P. — Yes. An instrument made in the manner I 
have described is termed an astronomical telescope. It is 
used only for looking at the heavenly bodies, because the 



208 OPTICAL INSTRUMENTS. 

images are shown inverted. To make the objects appear 
upright, two other eye glasses are used, of the same focus, 
in which the rays of light are made to cross one another, 
and the object is thus seen in its natural position. 

The most powerful telescopes are the reflecting ones, in 
which the image of the object is formed by reflection from 
a concave mirror, and it is afterwards viewed by an eye 
glass of small focal length. All telescopes, however, de- 
pend upon the principle of first forming an image of the 
object looked at, and then viewing that image by a lens of 
small focus ; and the larger the image, and the nearer it 
can be looked at, the greater will be the magnifying power. 

Frederick. — Then by forming a very large image, and 
using a very small eye glass, a telescope might be made to 
magnify to any size, I suppose 1 

Mr. P. — Many practical difficulties arise in the con- 
struction of telescopes that limit their magnifying power. 
For instance, as you increase the size of the image, the 
quantity of light upon it is diminished, as I have before 
mentioned ; and, therefore, the diameter of the tube and 
the circumference of the object glass, or reflector, must be 
large in proportion, or the image would be too dim to be 
clearly seen. You will have some idea of the difficulties 
attending the construction of large telescopes, when I tell 
you that the tube of the large reflecting telescope made by 
Dr. Herschel, weighed many thousand pounds, and that 
the reflector itself weighed nearly one ton. The diameter 
of its tube was four feet ten inches, and its length forty 
feet : this telescope magnified about 6000 times. 

Robert. — But could not he have made a shorter tele- 
scope magnify as much by using an eye glass with a very 
short focus? 

Mr. P. — When eye glasses of very short focal length 
are used, the great refraction of the rays of light separates 



OPTICAL INSTRUMENTS. 



209 



them into their prismatic colors, and makes the images 
seem very indistinct. — Having now told you how objects 
are apparently brought nearer by telescopes, you will read- 
ily understand the effect of the compound microscope, 
which magnifies objects in the same manner. 

Robert.— How can that be, father ; for in microscopes 
the things we look at are placed close to the glass, and 
with telescopes we see things at a great distance 1 

Mr. P. — The only difference between them is, that in 
telescopes, as the rays of light proceed from distant objects 
nearly parallel, the image is formed in the focus of the ob- 
ject glass ; and, in microscopes, the image is formed be- 
yond the focus. 

Harriet. — How is that, papa? 

Mr. P. — The object glass in a microscope is of very 
small focal length, and the object to be viewed is placed 
nearly in its focus ; therefore the 
rays diverge so much on entering 
the glass, that it cannot concen- 
trate them into points until they 
have passed far beyond the focus of 
parallel rays. The image is, con- 
sequently, as much larger than the 
object as its distance from the glass 
is greater, as I stated to you yester- 
day ; but this sketch will, perhaps, 
impress the subject more upon your 
minds. The small object a b, 
[Fig. XXVII.] is placed so near the 
focus of the small lens, ln f that the 
rays from it are not brought to a 
focus until they have arrived at * m. 
As the rays from the two extremities of the object cross each 
other in the centre of the object glass, I n, the size of the 
18* 



Fig. XXVII. 




210 OPTICAL INSTRUMENTS. 

image must depend upon its distance from the lens; and at 
i m — which is ten times as far from the lens as a b — it will 
be ten times longer than the object. This image, when 
viewed by the naked eye, will appear, therefore, ten times 
the length of the object ; and when looked at through an 
eye glass, c d, that will enable you to see it ten times nearer, 
it will appear to be ten times ten, or one hundred times, 
the length.* 

Frederick. — Does the solar microscope magnify objects 
in the same way ? 

Mr. P. — Not exactly. In the compound microscope an 
enlarged image of the object is formed, and it is then 
viewed by an eye glass ; but, in the solar microscope, we 
look at the image itself, thrown on a screen. The light of 
the sun is first reflected by a mirror, on the outside the 
window-shutter, upon a lens that concentrates the light 
upon the object to be viewed, which is placed near the fo- 
cus of a smaller lens, as in a compound microscope : and 
the cross rays, diverging as they proceed, form an image 
on the screen, that is magnified in proportion to the dis- 
tance of the screen from the lens. Objects may, by this mi- 
croscope, be magnified to almost any size ; for so much 
light may be concentrated upon them, as to make their im- 
ages distinctly visible when magnified even 500 times in 
length, and therefore '250,000 times in surface. In this 
manner we become sensible of the existence of hundreds 



* Compound microscopes consist of two or more lenses ; one of the 
simplest forms is represented by Fig-. XXVII. A single microscope is 
merely a lens of any transparent substance, in the focus of which the objects 
to be examined are placed. Such a microscope, for temporary use. can be 
formed by making a small hole in a card, and inserting a globule of water 
in it. A drop of water on glass is a plano-convex lens ; and if the drop be 
minute and die glass clear, it will answer very well for a microscope. — 
Am. Ed. 



OPTICAL INSTRUMENTS. 211 

of living beings, invisible to the naked eye, moving in full 
activity in a drop of water as their little world. 

Harriet. — I remember you showed us some mites in 
cheese with your solar microscope, last summer, which ap- 
peared as large as crabs, crawling about the screen with 
crumbs of cheese in their mouths, that seemed large enough 
for a man's dinner. 

Mr. P. — The astonishing effects of the magic lantern 
are produced in nearly a similar way to those of the solar 
microscope, the light of a lamp being employed as a substi- 
tute for that of the sun. 

Frederick. — I wish you would describe it to us, father. 

Mr. P. — Yes, my dear, I will ; for though it closely re- 
sembles the solar microscope in principle, yet, as the light 
is derived from a radiant point, and, therefore, strikes upon 
the objects divergingly instead of parallel, as in the solar 
microscope, an additional lens is necessary to counteract 
the effect of this divergency. Many of the descriptions, 
that I have seen of this instrument in elementary works, 
are very incorrect and unsatisfactory ; I will, therefore, en- 
deavor to make you clearly understand the mode of its 
operation. If you hold a painted slide close to the flame 
of a candle, it will intercept part of the diverging rays, and 
a large indistinct colored image will be thrown on the wall. 
If you were to place a convex lens a little beyond its focal 
distance from the slide, a distinct image of the central part 
of the painted figure will be formed ; and it will be as much 
larger than the corresponding part of the object as the dis- 
tance of the lens from the wall is greater than its distance 
from the object. This is a magic lantern in its simplest 
form. 

Robert. — What should prevent the whole of the figure 
from being seen, as in the real magic lantern 1 

Mr. P. — It is the divergence of the rays as they proceed 



'212 



OPTICAL INSTRUMENTS. 




from the candle ; but I can best explain this by a drawing. 
Here, a b [Fig. XXVIII. ] represents the position of the 

painted slide in the 
Fig. XXVIII. magic lantern, c d the 

tube through which the 
light proceeds to the 
screen, and f the flame 
of the lamp, sending 
forth its diverging rays 
to the painted figures. 
You perceive that most of the rays, excepting those that 
pass through the central part, are obstructed by the tube, 
and that only the middle ones reach the lens, In; the 
images of those parts, therefore, would be alone converged 
to a focus on the screen in such a magic lantern. 

Robert. — Then how is it contrived that the lens should 
collect the rays from all parts? 

Mr. P. — A large and powerful convex lens is placed 
close to the object, by which the rays of light are greatly 
concentrated, so as to cause most of the rays from the 
figure to fall upon the second lens at the end of the tube. 

Frederick. — Then the second lens acts in the same 
way as the object glass in a solar and in a compound 
microscope ? 

Mr. P. — Yes, it does. It is placed a little beyond its 
focal distance from the object, so that each pencil of diverg- 
ing rays, flowing from different parts of the object, may 
be brought to a point considerably beyond the real focus 
of the lens ; and if a screen be placed at a proper distance, 
a distinct image of the painted figure will be seen. As 
the rays of light cross one another in the middle of the 
object lens, the image will increase in size in proportion 
to the distance of the lantern from the screen ; but as it 
increases in size, its brightness diminishes* because the 



OPTICAL INSTRUMENTS. 



213 



same quantity of light is then spread over a larger surface. 
— Here is a section of a complete magic lantern, showing 
the situations of the lenses, a and b, [Fig. XXIX.] and the 

Fig. XXIX. 




direction of the rays of light, from their first divergence to 
the painted glass, till they cross in the object lens, and 
again diverge to the screen. 

Frederick. — Then the magnifying effect of the magic 
lantern is owing only to the rays of light from the lamp 
being collected to a focus on the screen, after passing 
through the painted glass ? 

Mr. P. — That is all. When the lantern is removed far- 
ther from the screen, the image will become indistinct : 
and, to bring the rays to a focus, it will be requisite to move 
the object lens nearer to the painted slide, to increase the 
divergence of the rays, and thereby prevent them from 
concentrating until they arrive at the more distant screen. 
When the lantern approaches the screen, on the contrary, 
it will be necessary to diminish the divergence of the rays, 
by removing the lens farther from the object. As the rays 
from the object cross in the centre of the lens, of course 
the images are inverted ; but this is easily remedied by 
inverting the painted figures, which are then shown upright. 

Frederick. — What is the difference between the magic 
lantern and phantasmagoria ? 

Mr. P. — The exhibition called phantasmagoria is pro- 



214 OPTICAL INSTRUMENTS. 

duced by merely surrounding the objects, painted on the 
slides, with a perfectly opaque back ground, and then 
throwing the images on a screen through which thev may 
be seen on the other side ; the exhibitor and the lantern 
being thus concealed from view, and nothing visible but 
the luminous figures. As the lantern recedes from the 
screen, the objects enlarge, and seem to advance upon the 
spectator ; and as the exhibitor brings the lantern nearer, 
they seem to depart. The effect is still more wonderful if 
the image be thrown upon smoke, rising from a concealed 
tire ; in which case, the moving smoke appears to give 
motion to the figure ; and, to a person not aware of the de- 
ception, it produces an appalling sensation. The image 
may also be reflected from a thick fog, as from a screen. 
Many a ghost story owes its origin to deceptions of this 
kind. 

Harriet. — Even when I know what it is, I cannot help 
feeling afraid when you exhibit some of the horrible figures 
in the magic lantern. 

Mr. P. — It is very foolish of you, Harriet. You should 
endeavor to conquer such weakness : and to give vou an 
opportunity of doing so, I will exhibit the magic lantern this 
evening ; it will be an agreeable termination to the subjects 
of light, lenses, vision, and optical instruments. 

Harriet. — Oh do, dear papa ! I shall be so much 
pleased if you will. 

Robert. — And if you will let me, father, I will paint 
round the figures on some of the slides with black, so that 
we may see the effect of phantasmagoria. 

Mr. P. — Yes, my dear, if jou wish it: and then we 
shall put Harriet's courage to the greater test. 



OPTICAL INSTRUMENTS. 215 



QUESTIONS. 

1. What instrument is used for examining distant objects ? 

2. What kind of telescopes is most commonly used ? — Describe it. 

3. What are the principal glasses in a refracting telescope ? — 
What is their use ? 

4. Explain Figure 26. 

5. How are objects represented by an astronomical telescope ? 

6. By what means may they be made to appear erect ? 

7. What other kind of telescope is used ? 

8. In what respects do a reflecting telescope and a refracting 
telescope differ ? 

9. By what circumstances is the magnifying power of telescopes 
limited ? 

10. Give some account of Dr. Herschel's telescope. 

11. What inconvenience attends the use of an eye glass with a 
very short focus ? 

12. For what purpose are microscopes used ? 

13. How does a telescope differ from a microscope ? 

14. What is a single microscope ? 

15. Explain Figure 27. 

16. Describe the solar microscope. 

17. How are objects represented by a solar microscope .? 

18. What is the most simple form of the magic lantern ? 

19. How are objects represented by it ? 

20. Explain Figure 28. 

21. How is the imperfection, indicated in this figure, avoided ? 

22. To what is the magnifying effect of the magic lantern owing ? 

23. Can the images produced by the magic lantern be magnified 
without being made indistinct ? — How ? 

24. How are the images exhibited in what is called phantasma- 
goria ? 



216 CONCAVE AND CONVEX MIRRORS. 



CONVERSATION XXIV 

CONCAVE AND CONVEX MIRRORS. 



Frederick. — Are the figures we see in the air, before 
a concave mirror,* produced by reflection, in the same 
way as the images behind a looking glass 1 

Mr. P. — Yes, they are. The effect is very extraor- 
dinary, and I will endeavor to explain the cause of it. 

Suppose e li [Fig. XXX.] 
Fig. XXX. to be a concave mirror, 

forming part of the interior 
of a globe, the centre of 
which is c. Now all lines 
that are drawn from the 
centre of a circle to its 
circumference are perpen- 
dicular to that point of the 
circumference to which they are drawn. The surface of 
a concave mirror may, therefore, be considered as com- 
posed of innumerable small plane mirrors, so arranged that 
lines of equal length, drawn from the centre of a circle, 
will fall perpendicularly upon them. If, then, rays of light 

* The student must not expect to see the effect here described, in the 
ordinary concave mirrors to be found in the shops. To exhibit the image 
in the air, before the glass, the mirror must be large and very accurately 
ground.— Am. Ed. 




CONCAVE AND CONVEX MIRRORS. 217 

issue from the centre c, and fall upon the concave surface 
of the mirror e h, all the rays will be reflected back upon 
the same point. 

Frederick.— Just in the same way that rays, striking 
perpendicularly against a looking glass, are sent back to 
the object ? 

Mr. P. — Yes ; and at the centre, c, an image of the 
object would be formed by the rays thus reflected meeting 
there. When the rays proceed from an object so distant 
that they arrive at the mirror parallel to one another, as 
d g b, they will be reflected at an equal angle on the other 
side the perpendiculars of the surfaces against which they 
strike. Thus the ray d e will be reflected in the direction 
e f, which is at an equal angle on the other side of the 
perpendicular c e. The ray 6 h will be also reflected to 
f; and all the other rays will, in the same manner, be 
concentrated at that point ; which is, therefore, termed the 
focus of such a mirror. The focus of parallel rays is half 
way between the centre of concavity and the mirror. At 
that point, the image of a distant object will be formed, 
and may be seen in an inverted position in the air. If you 
stand before this concave mirror, beyond the centre of its 
concavity, you will see yourselves in the air. (Mr. Powell 
places a concave mirror on the table, into which the children 
look; and they express great astonishment at seeing them- 
selves in the air with their feet uppermost.) 

Harriet. — What makes us appear to be upside down, 
papa? 

Mr. P. — I have hitherto considered the object to be a 
luminous point placed in the axis of the glass ; that is, in 
a line, c a, [Fig. XXXI.] drawn through the middle of it 
to the centre of concavity, c. But let us now trace the 
reflection of the rays from an object placed below the axis. 
We have here a representation of Robert standing before 
19 



216 CONCAVE AND CONVEX MIRRORS. 

the concave mirror, and below its axis. Rays of light 
must proceed from his body in all directions to the 

Fig. XXXI, 




mirror ; but we will only follow three of those proceeding 
from his head and his feet ; and as all the other rays, 
when converged to a point, collect around their central 
ones, we will first trace the reflection of the ray b a, as 
the other rays from the same point will collect on the 
line of its reflection. The ray b a, therefore, striking 
against the point a, to which the axis, c a, is perpendic- 
ular, will be reflected at an equal angle on the other side : 
that is, above the axis, in the direction a i. The rays b e 
and b d will be reflected in the same way, and meet on the 
line a i, at i, where an image of Robert's feet will be 
formed. The rays proceeding from his head, by obevin^ 
the same law of reflection, will be brought to a point on the 
axis of the mirror at m, and all the intermediate rays from 
his body will be reflected in their proper positions between 
m and z, forming there an inverted image of his whole 
figure. 

Robert. — But my little image is not in the focus, for it 
is nearer to the centre than it ought to be. 

Mr. P. — Your figure is represented as being so near the 
mirror, that the rays diverge as they proceed to it. Rays 



CONCAVE AND CONVEX MIRRORS. 219 

that diverge when they fall upon a concave mirror, cannot 
be brought to a point so soon as parallel rays ; and the 
greater the divergence the more distant will the image be 
from the focus, as is the case with images formed by convex 
lenses. Thus, if the object be placed at the centre of con- 
cavity-j the image will be formed there also, and their sizes 
will be equal. If the object be placed still nearer, the 
image will recede farther from the glass, and be larger 
than the object. For instance, if you were placed as near 
to a large concave mirror as your image is represented to 
be in the drawing, you and your image would then change 
places and sizes, and in that case it would be as much 
larger than you, as you are here drawn larger than it. — 
This property of concave mirrors of forming images in the 
air has been ingeniously applied to produce many deceptive 
appearances, calculated to strike the uninitiated with the 
greatest astonishment, and that have been regarded by the 
ignorant as the effects of magic. 

Robert. — When I look nearer to the concave mirror, I 
see my face behind it, and very much magnified. 

Mr. P. — Yes, Robert, that is another peculiarity of 
concave mirrors. When an object is placed in the focus 
of such a mirror, the incident rays diverge so much that 
they are reflected from it parallel to one another, and no 
image is formed. If the object be brought nearer than the 
focus, the rays diverge after they are reflected, though not 
so much as before. Now, persons cannot usually see any 
object distinctly, if placed nearer to the eye than six inches, 
owing to the great divergence of the rays from a near 
object ; but as a concave mirror reflects the rays in a less 
diverging direction than they were when they fell upon it, 
the image reflected can be looked at nearer than the object 
can with the naked eye ; and the nearer you can look at 
any thing, the larger it appears, as I have previously ex- 



220 



CONCAVE AND CONVEX MIRRORS. 



plained.* The subject will be more intelligible, perhaps, 
by means of this drawing. Let a b [Fig. XXXII.] repre- 
sent Robert's eye looking at itself in the concave mirror, 

Fig. XXXII. 




c d. The parallel rays, a c and b d, from the eye-brow and 
from the lower part of the eye, will be reflected into the 
pupil of the eye at e, and the image will be seen in the 
direction of the reflected rays i e and m e, viz. at i m, 
where the magnified image will be visible. 

Harriet. — What is the reason of every thing appearing 
so little in the round mirrors that we sometimes see in 
drawing-rooms 1 

Mr. P. — They are called convex mirrors, and bulge 
outwards in the middle, instead of being hollowed, as con- 
cave mirrors are. The effects produced by reflection from 
their surfaces are, as you must have observed, totally differ- 
ent from those produced by reflection from a concave 
surface. Parallel rays of light, falling upon a convex 
mirror, are made to diverge, as if proceeding from a 
point behind the mirror, as will appear from this figure. 
The parallel rays, a b and r d, [Fig. XXXIII.] proceed 
from the object a r to the convex mirror b d, which has its 
centre of convexity at c ; from which the surface of the 



* Conversations on Vision and Magnifying glasses. 



CONCAVE AND CONVEX MIRRORS. 



221 



convex mirror is drawn. Lines drawn from c to the points 
b and d will, therefore, be perpendicular to the surface of 




the mirror at those points, and the incident parallel rays 
will be reflected at an equal angle on the other side of 
them, that is, in the diverging directions b g and d k. 

Frederick. — Then such rays will never meet at a 
focus? 

Mr. P. — No ; they will, on the contrary, diverge more 
and more, the farther they extend. 

Harriet. — How does the diverging of the rays make 
things appear less than they are ? 

Mr. P. — I will show you how that effect is produced, 
by tracing the reflection of other rays from the object, a r, 
to the eye placed at e. It is evident, as the rays are re- 
flected in all directions, that the ray a n, in that figure, is 
the only one reflected to the eye from the point of the 
object, and r s the only one from the bottom of it. The 
image will consequently be seen in the direction of the 
reflected rays n e and s e, and will appear to be much 
diminished in size, as at i m* 



The rays of light reflected from a convex mirror, diverge as if proceed- 

19* 



222 



CONCAVE AND CONVEX MIRRORS. 



Robert. — I do not yet understand why things should 
appear less in a convex mirror than in a common looking 
glass. 

Mr. P. — It is owing to the convex surface reflecting the 
rays from points nearer to each other. Thus, suppose the 
straight line b d [Fig. XXXIV.] to be a plane mirror ; the 




ray from the point a of the object will be reflected to the 
eye at e from p, and the ray from r will be reflected from 
q, and the image will appear at i m of the same size as the 
object would appear if viewed from the other side the glass 
at o, because the angle peg and the angle p o q are equal. 
But when the same object is reflected from a convex sur- 
face, represented by the dotted curved line, the reflections 
from the top and bottom of it will take place from points 
nearer than, before, viz. from n and 5. The image is there- 
fore reflected from the reduced space of n s, instead of from 
p q; and it will appear, consequently, less than the object, 
as at i m. The angle subtended to the eye, by the reflec- 
tion from the convex surface, is, you perceive, much less 
than that of the reflection from the plane mirror ; and the 
difference, in apparent size, of the two reflections, will bear 

ing from an image placed behind the glass, between the centre of convexity 
and the convex surface ; but the apparent distance of reflected objects is 
generally far beyond the virtual focus of the mirror. 



CONCAVE AND CONVEX MIRRORS. 223 

the same proportion as the space between p q bears to the 
space between n s. 

Robert. — I see : the ray that strikes against the glass 
at the point p, on the convex surface, would be reflected 
above the eye, because it would be ne; r.y in the line of the 
perpendicular of the convex surface at that part. 

Mr. P. — You in.' right : no ray from the point of the 
arrow, but that reflected from n, would be seen by the eye 
at c; for all the other rays would be reflected above or 
below it. 



QUESTIONS. 



1. What is the form of a concave mirror? 

2. How are rays, which diverge From the centre of a concave 
mirror, reflected ? 

3. How are parallel rays reflected ? 

4. What is the focus of a concave mirror ? 

5. Where is the focus of parallel rays ? 

6. Explain Figure 30. 

7. How is an image represented by a concave mirror ? 

8. Why does it appear inverted ? 

9. Explain Figure 31. 

10. When does the image, presented by a concave mirror, appear 
smaller than the object ? — When of the same size ? — When larger ? 

11. How do you account for the magnifying power of concave 
mirrors ? 

12. Explain Figure 32. 

13. What is the form of a convex mirror ? 

14. Are objects seen in a convex mirror magnified or diminished ? 

15. How are parallel rays reflected by convex mirrors ? — Diver- 
gent rays ? — Convergent rays ? 

16. Explain Figure 33. 

17. Do convex mirrors bring rays to a focus ? 

18. Why do things appear less in a convex mirror, than in a 
looking glass ? 

19. Explain Figure 34. 



224 THE KITE. 



CONVERSATION XXV 



THE KITE. 



Mr. P. — As the morning is remarkably mild and fine, 
we will take a walk ; and, Frederick and Robert, bring 
your kites, that, as you fly them, I may endeavor to explain 
the cause of their ascent. (Frederick and Robert fetch 
their kites, and accompany Mr. Powell and Harriet in 
their walk.) 

Frederick. — I should like very much to know what 
causes the kite, which is so much heavier than the air, to 
support itself at so great a height : it is a thing that has 
often puzzled me. 

Robert. — It must be owing to the wind, for we know 
that kites will not fly when there is no wind. 

Frederick. — Yes, Robert, I know that ; but how is it 
that the wind can* keep the kite in the air, for a length of 
time, when all other things, blown up by the wind, soon 
come down again ? 

Robert. — Why, it is owing to the tail and the string. 

Mr. P. — The tail and the string are, indeed, necessary 
to enable the kite to ascend ; but you should be able to tell 
in what manner they produce this effect; otherwise, your 
explanation conveys very little information. The cause of 
kites flying is a question that has been thought worthy of 
the investigation of the greatest mathematicians; but I 
shall try to make you understand it sufficiently, without 



THE KITE. 225 

reference to what would be, to you, puzzling demon- 
strations. 

Frederick. — Shall I get my kite ready now ? 

Mr. P. — The field we are in will do very well ; and this 
breeze is favorable to us. Let me look at your kite, Fred- 
erick, and see that it is properly balanced. 

Frederick. — Is that of much importance ? 

Mr. P. — The kite will not fly steadily unless the sides 
afe equally balanced ; and it is for this reason the wings 
are added, which are of no use, provided the kite balances 
without them. (Mr. Powell holds the kite up by the 
string, to try whether it balances, and whether the lower end 
dips down sufficiently.) It is all right, I perceive ; there- 
fore get your string ready : and, Harriet, hold the kite up 
against the wind to assist its ascent. (Frederick runs 
with the string, which draws the kite out of Harriet's hand, 
and it rises in the air.) Stop, Frederick, you need not run 
any farther, but let out the string gently, as the kite draws 
it through your fingers. 

Harriet. — What was the use of his running at all 1 

Mr. P. — The resistance of the air, as Frederick ran 
with the kite, acted in the same manner as the wind blow- 
ing against it, and therefore assisted the kite to rise in the 
air. The running is necessary, also, in the first instance, 
to keep the kite in an oblique position until the tail has 
cleared the ground. 

Frederick. — Is it absolutely necessary that the kite 
should be kept in a slanting direction to make it rise ? 

Mr. P. — The principle of kite-flying depends entirely 
upon it, as you will perceive on a slight explanation. Rob- 
ert, hold up your kite obliquely, at an angle of about 45°, 
while Harriet holds the string in a direction perpendicular 
to the plane of the kite. (Robert and Harriet hold the 
kite and string as requested.) As the wind blows against 



226 



THE KITE. 



the kite horizontally, while it is in this position, it forms an 
angle with its surface of 45°, and will be reflected at an 
equal angle on the other side of the perpendicular, repre- 
sented by the string. Now, you are aware, when any thing 
is reflected from the surface of another, the reflecting body 
is acted upon with as much force as the tiling reflected ; in 
other words, the action and re-action are equal, and in op- 
posite directions. This is one of the laws of motion. The 
kite will, therefore, be acted upon by the reflection of the 
wind in the opposite direction to that in which the wind is 
reflected ; and, as Robert's kite is now held inclining 
at an angle of 45°, and the wind strikes it horizontally, the 
reflection will be perpendicular to the horizon, and the re- 
action on the kite, that is, the force of the reflected wind, 
will be directed perpendicularly upwards. 

Harriet. — I do not exactly understand what you mean. 
Could you make the explanation clearer by showing us 
the direction and reflection of the wind in a drawing? 

Mr. P. — I will endeavor to do so, and if I succeed in 
making you comprehend the theory of kite-fiying, you 

will know more on the 
Fig. XXXV. subject than the most 

irr-- — -,& successful kite-flyers 

generally do. Let a b 
[Fig. XXXV.] repre- 
sent Frederick's kite 
in the air, inclined at 
an angle of 45° to the 
surface of the earth, 
and let d s represent 
the string, which we 
will suppose to be per- 
pendicular to the plane of the kite a b. If the wind be 
blowing in the direction w d, when it strikes the kite at d 




THE KITE. 227 

(forming the angle w d s with the perpendicular), it will 
be reflected at an equal angle on the other side the 
perpendicular d s, that is, in the direction d g; and the 
force of the reflected wind, re-acting on the kite in the 
opposite direction, will tend to carry it perpendicularly 
towards h. But the wind, in the direction w d, also acts 
on the kite at the same time, tending to carry it horizon- 
tally towards k, and the weight of the kite itself is tending 
to bring it down to the ground in the perpendicular direc- 
tion d g. The kite is thus acted upon by three forces, — 
one impelling it towards k, the other towards*^, and the 
third towards g. 

Harriet. — Then the poor kite is pulled three ways at 
once, — upwards, downwards, and sidesways. It must be 
puzzled, I should think, to know which way to go. 

Mr. P. — It takes a direction between the three. If the 
weight of the kite pull it towards the ground with the force 
of two pounds, and if it be impelled horizontally with a 
force equal to two pounds, and upwards also with the 
same force, the kite will move horizontally, and the two 
other forces will be destroyed. 

Frederick. — How is that, father ? 

Mr. P. — Because the forces in the opposite directions, 
d g and d h, act directly against each other, the one up 
and the other down, with equal strength, and therefore have 
no impelling power up or down, and leave the kite to be 
propelled solely by the horizontal power, which will act 
upon the kite with its whole force of two pounds. If the 
forces be unequal, that is, supposing the force of the wind 
horizontally and upwards to be four pounds each, instead 
of two, while the weight of the kite remains the same, 
then it would be impelled horizontally towards k, with a 
force pf four, and upwards with the force of two pounds 
(which is the difference of power between the upward and 



228 THE KITE. 

downward forces). If we make the dotted line d k twice 
the length of d h, that being the proportion of their respec- 
tive forces, and draw the parallelogram d h I k, then the 
diagonal d I will represent the direction of the kite under 
such circumstances. If the string of the kite be held tight, 
its horizontal motion is thereby prevented, and the whole 
force of the wind is directed upwards. 

Frederick. — Then do the forces vary in power accord- 
ing to circumstances ? 

Mr. P. — Yes; the reflective force varies with the re- 
sistance offered to the wind by the reflecting body. For 
instance, if you were to slacken the string, the resistance 
of the kite to the wind would cease, and the kite would be 
carried along in the direction of the wind, till it fell to the 
ground ; for as soon as the resistance ceased, the reflection 
would cease also. You can now try the experiment. Let 
the string run out rapidly, and by that means you will 
diminish the kite's resistance to the wind, and, conse- 
quently, lessen the reflecting power. (Frederick lets the 
string run out, as his father directs, and the kite begins to 
descend.) 

Harriet. — Look, Frederick, your kite is falling. 

Mr. P. — It has lost the reflective force of the wind, in 
consequence of the string being loosened, and can no 
longer support itself in the air. Pull the string in quickly, 
Frederick, or it will be down : — quick ! quick ! 

Frederick. — (P idling in the string.) Now it is rising 
again, and more perpendicularly than before. 

Mr. P. — Yes, you have pulled the string in so tightly, 
that the horizontal force of the wind cannot act upon the 
kite ; and the reflective or perpendicular force is therefore 
brought into action with scarcely any counteracting power. 

Robert. — What is the use of the kite's tail ? 

Mr. P. — It has two very important uses : in the first 



THE KITE. 229 

place, its weight acts as ballast, and keeps the kite in an 
upright position by bringing the centre of gravity below the 
point of suspension. 

Robert. — That might be done by fastening a small 
weight to the bottom. 

Mr. P. — It might so, but not nearly so well ; for it 
would require a much heavier weight to answer the pur- 
pose, if fixed to the bottom of the kite, than it does when 
fastened at the extremity of a long string ; and it is im- 
portant to have the kite as light as possible. The tail 
also serves another most essential purpose, viz. to preserve 
the proper inclination of the kite. 

Frederick. — In what manner does it do that, father 1 

Mr. P. — The " bobs," or pieces of paper fastened to the 
kite, present a considerable surface to the action of the 
wind : and if the weight at the end be not too heavy, the tail 
will be carried out by the wind, and will pull the kite into a 
slanting position, which, as I have shown you, is essential 
to its rising. 

• Robert. — I will try to fly my kite without the bobs, and 
see how it will do. 

Mr. P. — Do so, Robert ; there is nothing so convincing 
as experiments. (Robert takes off the bobs from the tail, 
leaving the weight at the end.) Harriet, hold it up, and let 
the kite with a naked tail have a fair trial. Now, Robert, 
run with it as fast as you can. 

Harriet. — It scarcely rises high enough to clear the tail 
from the ground ; and, now Robert stops, it is fallen. 

Mr. P. — I concluded it would do so ; for the tail hangs 
so perpendicularly without the bobs, that the kite cannot 
maintain its proper inclination. (Robert comes back with 
his kite.) Well, Robert, are you satisfied that the papers 
are of use ? 

Robert. — Yes; I must fasten them on again, for the 
20 



230 



THE KITE. 



kite flew nearly upright without them. What would be the 
effect of increasing the number and size of the bobs ? 

Mr. P. — In that case the wind would carry the tail out 
too much, and the kite would be inclined too horizontally. 
Frederick. — At what angle should a kite be inclined, 
to fly best ? 

Mr. P. — You perceive, from the drawing, that, at an 
angle of 45°, the reflected wind acts most perpendicularly ; 
but a position rather more upright than that would have a 
better effect in raising the kite, as a larger surface would 
be presented to the action of the wind ; and though the 
kite, in that case, would rise more horizontally, yet the ad- 
vantage of having a larger surface exposed to the wind 
would more than compensate for any disadvantage in the 
loss of perpendicular force. The inclination of a kite may 
be greater in a strong wind than in a gentle breeze. 

Frederick. — There is one thing, father, that takes place 
when a kite is flying, that must make an alteration in the 
reflection of the wind. You spoke of the kite as having a 
flat surface, but we know that, when it is in the air, the 
wind blows the two sides very much backwards : what 
effect has that on the reflection of the wind? 

Mr. P. — I am glad you have reminded me of the cir- 
cumstance, though it does not al- 
ter our theory of kite-flying, as 
depending on the re-action of the 
reflected wind. The inclination 
of the sides of the kite produces 
an alteration in the direction of 
the reflected force of the wind, so 
as to bring it to bear more at right 
angles upon the surface of the kite, 
as you may perceive from this 
drawing. The lines k k [Fig. 
XXXVI.] represent the sides of 




THE KITE. 231 

the kite bent backwards by the wind ; and the dotted lines, 
r r, the directions in which the re-action of the reflected 
wind takes effect upon the inclining sides. The forces, in 
this case, are not directed perpendicularly upwards; but 
the re-action, in this position, operates much more effectu- 
ally, as it is exerted at right angles to the kite, instead of 
acting obliquely against it, as before. Besides, as the re- 
flected forces act on each side, at an equal angle from the 
perpendicular, the result of the composition of the two 
forces will be a perpendicular impulse in the direction s d. 

Robert. — Would kites fly better if they could be made 
quite stiff and unyielding ? 

Mr. P. — No, my dear, not so well by any means ; for 
the acfion of the wind, in different directions, on the in- 
clined sides of the kite, tends to keep it steady, and pre- 
vents the kite from turning edgeways to the wind, which it 
would be apt to do if the surface were quite flat. A very 
simple experiment will show the advantage of inclining 
sides in steadying the kite. I will fasten a piece of string 
to the middle of this card, which is quite flat, and Harriet 
shall run with it as if she were flying a kite. (Mr. Powell 
fixes the string to the card, and then gives it to Harriet, 
who runs with the card, lohich moves through the air very 
unsteadily.) 

Robert. — Harriet's kite wriggles about in all directions. 

Mr. P. — That is owing to the wind acting against it in 
different directions every instant, in consequence of the 
card not being kept sufficiently steady by the string, to pre- 
serve it in the same inclination as Harriet runs with it. 
Stop, Harriet ; bring back your kite to me, and I will make 
it fly more steadily. 

Harriet. — (Coming back with the card.) I wish you 
would, papa; for it is all in a whirligig at present. 

Mr. P. — I will bend the card in the middle, lengthways, 



232 THE KITE. 

so as to resemble a kite when bent by the wind ; and you 
will find that now the action of the air, against the two sur- 
faces, as you run, will steady the card. There, run, 
Harriet. 

Harriet. — (Running with the card, which now moves 
steadily.) Look, Frederick, at my kite now; it is flying 
almost as well as yours. 

Frederick. — It falls, though, when you stop ; while 
mine has kept up all this time without my moving. 

Mr. P. — Yes, Harriet's kite is too heavy, for the size of 
it, to remain in the air without the action of a very brisk 
wind. The running, and the force of the wind together, 
kept it up ; though, when she stops, it falls. But come, 
Frederick, pull in the string of your kite, and we will re- 
turn home. I have explained to you, I hope satisfactorily, 
the principle on which the ascension of kites depends. 
There are, indeed, many phenomena, connected with this 
subject, that merit farther attention ; but they all depend 
upon, and may be explained by, the reflection of the wind 
from the inclined surface of the kite, occasioned by the re- 
sistance of the string. When your kite suddenly rises 
higher without your pulling the string, it is owing to an in- 
creased velocity of the wind increasing the reflecting force 
on the kite ; when the kite is falling, the wind must have 
abated or changed its direction. You may, upon the same 
principle, account for all the changes you may observe in 
the motion of the kite. 

Frederick. — The kite seems to be quite a philosophical 
instrument. 

Mr. P. — It is indeed ; and you may, when amusing your- 
self with it, be very philosophically employed in finding out 
the causes of its evolutions in the air. 



THE KITE. 233 



QUESTIONS. 

1. What is the use of a boy's running when he would raise 
his kite ? 

2. In what position should a kite be kept ? 

3. If a kite be inclined to the horizon at an angle of 45°, how will 
the wind be reflected from it ? 

4. What will be the effect of this reflection ? 

5. What is the law of action and reaction ? 

6. Explain these principles by Figure 35. 

7. By how many forces is the kite acted upon ? — What are they ? 

8. What direction must the kite take ? 

9. If the upward and downward forces are equal, how will the 
kite move ? — Why so ? 

10. If the force upwards is twice as great as that downwards, how 
will it move ? 

11. Why does a kite fall when the string is slackened? 

12. What is the use of the kite's tail ? 

13. Will a kite fly without " bobs ?"— Why not ? 

14. What will be the effect if the size and number of the " bobs" 
be increased ? 

15. At what angle should a kite be inclined, to fly best? 

16. What principle is illustrated by Figure 36 ? — Explain it 

17. How do the inclined sides of a kite tend to keep it steady ? 

18. Can you account for the motion of a windmill? 

19. In what manner is a vane made to indicate the direction of 
the wind ? 

20. Of what use are the feathers in an arrow and a shuttlecock ? 

20* 



234 SAILING. 



CONVERSATION XXVI 



SAILING. 



Mr. P. — When we were at the sea-side, last summer, 
you must have noticed the ships and boats sailing in oppo- 
site directions at the same time, though the wind was 
blowing upon them all from the same point. Do any of 
you know by what means ships are made to sail against the 
wind 1 

Robert. — It is the rudder, by which the sailors turn the 
ships in the way they want them to go. 

Mr. P. — The rudder will do much ; but it will not do 
all : and no ship could keep at all to the windward by 
means of the rudder alone. But can you tell us how the 
rudder acts in altering the direction of a ship ? 

Robert. — All I know is, that when the rudder is turned 
one way, the ship is turned the contrary way ; and I sup- 
pose it is owing to the resistance of the water. 

Mr. P. — So far, so good ; but we must proceed a little 
farther, to be able to understand the cause of this effect. 

Harriet.- — Yes, but you said the rudder alone would 
not make a ship sail against the wind: what is it, then, 
that does ? 

Mr. P. — It depends upon the position in which the sails 
are placed to receive the wind, aided by the action of the 
rudder ; but as the effect of the latter is most obvious, I 
will first explain its cause before I speak of the sails. 



SAILING. 235 

Frederick, your little model of a steam-boat will be of 
great use to us in illustrating the subject ; therefore fetch 
it, and we will all go to the pond and give it a trial. 
(Frederick brings the boat, and Mr. Powell and his 
children repair to the pond.) 

Harriet. — I shall be glad to see Frederick's little 
boat set to work ; it looks so very pretty making its way_ 
through the water by itself. 

Mr. P. — It has, indeed, a very pleasing effect. The 
paddles are turned by a spring, similar to that of a watch. 
You perceive, now that I have wound it up, they turn 
round briskly. I will place the boat on the water, with 
the rudder at liberty, in the first instance, and it will cross 
over in a straight line towards the tree opposite, in which 
direction I will point the head. (3Ir. Powell launches 
the boat on the water, and the working of the paddles car- 
ries it directly across the pond.) 

Harriet. — There it goes, pretty thing ! straight across. 

Mr. P. — Bring it to me, Frederick, and I will next tie the 
rudder on one side, and we shall see its effect in changing 
the direction of the boat. (Frederick fetches the boat, 
and gives it to his father, who ties the rudder, and then 
places it on the water in the same direction as before, with 
the paddles working.) You observe that the boat, instead 
of going towards the tree opposite, is turning its head 
round to the same side as that on which the rudder is in- - 
clined, and it is coming back to this side of the pond. 

Robert. — I said that the rudder would turn the ship 
round. 

Mr. P. — You did so, Robert ; and now that we have 
seen the effect, let us find out the cause. You, Robert, 
have given us your explanation, and attribute it to the re- 
sistance of the water. Are you, Frederick, prepared to 
tell us how the resistance of the water operates on the 
rudder, so as to turn the boat 1 



236 SAILING. 

Frederick. — Yes, father, I think I see how it is. As 
that side of the boat, towards which the rudder is turned, 
must meet with more resistance from the water than the 
other side, it cannot, in consequence, move so fast ; therefore, 
the other side, which meets with no such obstruction, by 
moving quicker, must turn the boat round, in the same 
way that a carriage turns round when one horse trots faster 
than the other. 

Mr. P. — Very well explained, indeed, Frederick. I 
did intend to mention to you in what manner the water 
acts upon the rudder to cause its resistance ; but your ex- 
planation will, perhaps, be sufficient for our present purpose, 
and we will next proceed to hoist our sails. I will let the 
rudder continue tied, as in the last experiment ; though I 
shall not now use the paddles, but trust only to the wind, 
which is blowing from east to west, along the pond. I will 
place the sails in an oblique position to the wind, and again 
launch the boat towards the tree, which is nearly north, 
therefore the wind blows across the vessel. (As the wind 
strikes against the sails, the boat begins to sail across the 
pond, and arrives on the opposite bank, a little to the ?cest- 
ward of the tree.) 

Robert. — When put on the pond before, with the 
rudder tied in the same way, and the paddles working, the 
boat went quite round towards the east ; but look ! the 
wind has taken it right across, and rather to the west of 
the tree. 

Mr. P.— Well, Robert, you must now be convinced 
that it is not the rudder alone that alters the course of a 
ship; for you have seen the boat carried in opposite direc- 
tions, with the rudder fixed in the same place each time. 

Robert. — Yes, I see that the sails can alter the direc- 
tion t03. Can you explain to us, father, how this effect 
is produced by inclining the sails to the wind ? 




SAILING. 237 

Mr. P. — I will endeavor to do so, Robert, in such a 
way that, by paying a little attention, you may be able to 
understand the cause of ships sailing against the wind. 

This drawing will assist in 
Fig. XXXVII. making the subject more 

. ? <rt clear : a b [Fig. XXXVII.] 

\ represents the boat, b c the 

rudder, s s the sail, and e w 
the direction of the wind 
from east to west. The 
wind, in this position of the 
sail, strikes upon it in an 
oblique direction, and is re- 
flected from it at an equally 
oblique angle, in the direc- 
tion w b. The reflected 
wind, therefore, reacts upon the sail in the direction from b 
to w, with nearly equal force to that of the wind blowing from 
e to w. The vessel being thus acted upon, at the same time, 
by two nearly equal forces, (the one in the direction from e 
to w, and the other in the direction from b to ic,) is inclined 
to take a course between the two, towards d ; and it would be 
propelled in that direction were the head of the boat pointed 
towards it. The water, however, resists the motion of the 
boat in that course, so long as the head is kept pointed towards 
n ; and it is obliged to proceed in the direction which offers 
least resistance to its progress through the water ; that is, 
with its head foremost. As the moving power is, however, 
exerted in the direction from w to d, the vessel will make 
some way in that direction, and the real course of the 
vessel will be w g * 



* This deviation, sideways, from the direct course of the vessel, is called 
its lee-ioay. — Am. Ed. 



238 SAILING. 

Robert. — But the boat must move sideways, then, if it 
go from to to g, while the head remains pointed towards n. 

Mr. P. — It does, indeed, move rather obliquely ; but 
you must remember, that its natural course is the very 
oblique direction to d ; and it is only owing to the resist- 
ance which the water offers to the boat moving sideways, 
that it does not take that course towards which it is impelled. 
As the resistance offered to the progress of the boat, in 
an oblique direction, gradually diminishes as the direction 
approaches nearer to that of the head of the boat, the extra 
resistance will, at a certain point, be overcome by the 
direction of the impelling power, and the vessel will sail in 
that course. 

Frederick. — Then I suppose that the greater the re- 
sistance of a boat in moving sideways, or obliquely, the 
nearer it will be able to sail against the wind ? 

Mr. P. — You are right. The keel at the bottom of 
ships tends greatly to produce this effect, as it presents a 
direct resistance to the motion of the vessel sideways. 

Robert. — How is it that the head of the ship does not 
turn from the wind, and so let the ship go in the course 
that offers the least resistance 1 

Mr. P. — The rudder prevents it. The principal use of 
the rudder, in sailing, is to keep the head of the ship in its 
right course, that the sails may be properly adjusted to 
catch and reflect the wind, so as to propel the vessel for- 
ward. Were it not for the rudder, ships would be blown 
in the direction of the wind, and be incapable of guidance. 

Robert. — Then it is the rudder, after all, that makes 
ships sail against the wind. 

Mr. P. — Ships could not sail against the wind without 
the aid of the rudder ; but the rudder of itself would do 
nothing towards keeping the ship in its course, unless the 
sails were properly placed. Suppose, for instance, that the 



SAILING. 



239 



Fig. XXXVIII. 

; >> 




sails of the ship, instead of being set as before, were placed 
in the contrary direction, across the vessel, as represented 
in this drawing, while the rudder remained in the same 
position ; the wind, blowing from e to w, [Fig. XXXVIII.] 
would be reflected towards r, 
and would react upon the sail in 
the direction from r to w. The 
sail being thus acted upon by 
two forces, one from e to w, and 
the other from r to w, the pro- 
pelling power would be in a 
direction between both, that is, 
from p to w y and the vessel 
would move backwards, with 
the stern foremost, till it turned 
round so as to bring the sail s s 
in a line with the direction of the wind, when it could no 
longer take any effect upon the sail, 

Harriet. — You say, papa, that ships can sail against the 
wind ; but I suppose they cannot sail directly against it ? 

Mr. P. — No, my dear, not directly ; for if the head of 
the ship were pointed directly against the wind, the ship 
would be blown backwards. By sailing against the wind, 
I mean, sailing in a direction different from that in which 
the wind blows ; as, for instance, sailing north when the 
wind is blowing from the east or west. When ships sail 
still nearer to the wind, they can make but little progress, 
as the wind must then strike the sails so obliquely as to 
have little effect upon them. 

Robert. — Of course a ship sails the fastest when the 
wind is blowing at its stern, for then the sails catch it all. 

Mr. P. — A vessel with only one sail might do so, but it 
is by no means the case in vessels with two or three masts ; 
for when the wind blows at the stern, the sails in front be- 



240 , SAILING. 

come almost useless, because the wind is taken from them 
by the sails nearest the stern. Such a ship sails the quick- 
est when it is just sufficiently inclined to the wind to admit 
of all the sails being acted upon at the same time. 

Frederick. — What do sailois mean by tacking 1 

Mr. P. — When the wind blows too closely in the direc- 
tion of a ship's course to allow of its sailing towards that 
point, it is made to sail as near to the course as possible, 
and is then turned back to start from a fresh point. In 
this manner the ship would at length arrive, in a zigzag 
direction, at the point of its destination. 

Harriet. — It must be very tiresome to go backwards 
and forwards in that manner. 

Robert. — Yes ; when you have just got in sight of land, 
to be turned back, and taken out to sea again, must be very 
provoking. 

Mr. P. — Well, my children, I think I have now told 
you sufficient respecting the general principles of sailing, 
to enable you to understand why ships should sail different 
ways with the same wind. With Frederick's little steam- 
boat you will be able to carry the principles of sailing into 
practice on a small scale ; and you will always find that it 
is the position of the sails which causes the impelling power 
of the wind to vary its direction; that it is the rudder ichich 
keeps the head of the ship in its proper direction ; and the 
resistance of the water to the motion of the vessel sideways, 
that causes it to be propelled nearly in the course towards 
which the head is pointed. 



SAILING. 241 



QUESTIONS. 

1. How can a ship be made to sail against the wind ? 

2. How is the course of a vessel affected by its rudder ? 

3. In what manner does the position of the sails influence the 
course of a ship ? — Explain Figure 37. 

4. What do you understand by lee-way? 

5. What is the principal use of the rudder ? 

6. Can a vessel be kept in its course by means of the rudder 
alone ? — How is this illustrated ? — See Fig. 38. 

7. Can ships sail directly against the wind ? 

8. Does a ship sail the fastest when before the wind ? — Why not ? 

9. What do you understand by tacking f 

10 Repeat the principles explained in this conversation. 
21 



242 FLYING. 



CONVERSATION XXVII 

FLYING. 



Harriet. — What a delightful thing it must be to be able 
to fly, like the birds ! Is it known, papa, how they are able 
to support themselves in the air ? 

Mr. P. — Yes. my dear : flying depends entirely upon 
the rapid action of the wings of birds upon the air. The 
muscles which move the wings of birds are exceedingly 
powerful ; and when the wings strike against the air with 
great force, the resistance they meet with is sufficient to 
raise the bird from the ground. 

Frederick. — What force would be sufficient to raise 
them'? 

Mr. P. — A power rather greater than the weight of their 
bodies. You may conceive that if a bird be able to lift its 
body by the strength of its wings, when resting them upon 
a firm support, a sufficient purchase, or hold, upon the air 
would only be requisite to enable the wings to raise it from 
the ground. This purchase birds obtain by the rapid mo- 
tion of their win^s. 

Robert.-— But all birds do not move their wings equally 
quick ; and the larger birds seem to move their wings more 
slowly than the little ones. 

Mr. P. — Y'es, they do; but the wings of large birds are 
proportionably larger, and therefore take much more hold 



FLYING. 243 

upon the air, and move through a greater space at each 
stroke. 

Harriet. — But some birds seem to fly without moving 
their wings at all. I cannot understand that. 

Mr. P. — It is only the birds with large wings that can 
do so ; and their apparently horizontal motion through the 
air depends upon the same principles as that of the sailing 
of the ships against the wind. When the bird expands its 
wings and is motionless, its weight tends to bring it down 
perpendicularly to the earth ; but the expanded wings and 
tail presenting great resistance to the air in that direction, 
the bird moves in the course which offers least resistance, 
viz. in that towards which its head is turned ; and it comes 
gradually to the earth in a very oblique course. In the 
same manner I explained to you the cause of a ship sailing 
in the direction towards which the head is pointed. Birds 
often seem, indeed, to move quite horizontally while their 
wings are motionless ; but this appearance is owing to our 
not being able to see, from below, the exact course of their 
flight. Their progress through the air, in such cases, de- 
pends entirely upon their gravitating force; that is, upon 
the power with which they are drawn towards the earth. 

Robert. — But I have seen birds, after darting down- 
wards for some time, rise in the air again without moving 
their wings. 

Mr. P. — You are determined, Robert, to puzzle me, if 
you can; but, perhaps, I shall puzzle you when I tell you, 
that the rising in the air you have noticed, depends upon 
the force with which the bird falls downwards. 

Harriet. — Well, papa, that is a puzzle indeed ! What 
do you think now, Robert 1 Will you give it up ? 

Robert. — Yes ; I think any body will find it a hard 
matter to make that out. 



244 FLYING. 

Mr. P. — I think a very satisfactory clew to the puzzle 
will be discovered, when I mention the Russian slides. 

Frederick. — Yes, I remember ; the force that sent the 
carriages down one hill carried them up another. Then is 
it in the same way that the force which the bird gains in 
falling down sends it up again 1 

Mr. P. — You have guessed right, Frederick. 

Robert. — In the Russian slides there was a hill made 
on purpose for the carriage to slide up ; but the bird has 
nothing to support itself upon. 

Mr. P. — You forget the air, Robert. 

Harriet. — But still, papa, I am puzzled to think what 
can make the bird alter its course, and suddenly rise, if it 
does not move its wings. 

Mr. P. — The principal cause of the change in the direc- 
tion of its flight is produced by the tail, which acts as the 
rudder of a ship in varying the course of the bird up and 
down. Some scientific men, I am aware, consider it to be 
a vulgar error to suppose that the tail of a bird acts as a 
rudder in changing its course ; but the vulgar opinion is, 
nevertheless, correct. It is true, the tail can have little 
influence in directing the horizontal motion ; but, in ascend- 
ing and descending, it is of the greatest utility. 

Frederick. — In what way does a bird move its tail to 
make it rise 1 

Mr. P. — We will suppose a bird to be descending rap- 
idly, at an angle of 25° to the horizon, and to have ac- 
quired the force it would have done in descending from a 
Russian slide of the same inclination. When it arrives at 
the bottom of the intended descent (during which the tail 
has been nearly in a line with the body), the bird points its 
head upwards, spreads out its tail, and raises it. The elevated 
tail presents considerable resistance to the air, and the body 



FLYING. 245 

of the bird is thereby pointed upwards, in the same 
manner as a ship is turned by the rudder. The direction 
in which the bird's body is then pointed being that which 
offers least resistance to its progress, it continues in an 
upward course so long as the impelling power (gained in 
the previous descent) overcomes the gravitating force, 
which tends to draw the bird to the earth. 

Frederick. — Then, I suppose, when a bird wants to 
descend, it inclines its tail downwards. 

Mr. P. — Yes ; that motion of the tail would bring its 
head in a direction more perpendicular to the earth, and it 
would, therefore, descend the faster. 

Robert. — If the tail direct birds only in ascending and 
descending, what is it that gives them their direction 
sideways 1 

Mr. P. — Their wings produce the alteration in their 
horizontal course. On turning to the left, a bird has only 
to move its head in that direction, at the same time striking 
more forcibly with its right wing, and the object will be 
accomplished : on turning to the right, the motions of the 
head and wing must be reversed. 

Frederick. — Just in the same way, I suppose, that we 
turn a boat, by pulling with one oar harder than with 
another. 

Mr. P. — Exactly the same. 

Robert. — If the air offers so much resistance to the 
descent of birds, when their wings are spread out, I should 
suppose that when they are once risen in the air, they can 
keep themselves there with very little labor. 

Mr. P. — Yes, my dear ; when they are risen as high as 
they wish, very little exertion is required to sustain their 
bodies at that height. When flying rapidly in a horizontal 
course, the lateral resistance of the air, and its reflection 
from their bodies, must be more than sufficient to support 
21* 



246 



FLYING. 



them. The whole of their strength may then be exerted 
horizontally. 

Frederick. — Does the resistance of the air, as birds 
move through it horizontally, help to keep them up ? 

Mr. P. — Yes. The effect produced by their motion 
through the air is similar to that of the wind blowing 
against them. I have shown you that, in the case of ships 
and kites, when the wind strikes against the inclined sails, 
or paper, it is reflected at an equal angle, and re-acts 
against the sails or kite in an opposite direction. Sup- 
posing this curved line, a b, [Fig. XXXIX. J therefore, to 

represent the breast of a 
Fig. XXXIX. bird flying horizontally 

through the air, w e would 
be the direction of the air's 
resistance, against which 
the point e is impelled. As 
p e is perpendicular to the 
curve at the point e, the 
air would be reflected at 
an equal angle on the oppo- 
site side of it, that is, in the 
direction e k; and the 
point e would be acted upon by two forces, — one in the 
direction from w to e, and the other from Jc to e. The real 
direction, from the composition of the two forces, therefore, 
would be the mean between the two, or from p to e. The 
same would be the case in all other parts of the curvature 
of the breast of the bird. Thus, you perceive, if the bird 
moved in the direction from e to w, it would receive an 
impulse upwards from the resisting force in front, as well 
as be supported by the resistance of the air beneath. If 
the motion were very rapid, birds would actually be raised 
in the air by the exertion of their strength in moving hori- 




FLYING. 247 

zontally ; as the impulse of the reflected air upwards would 
then be so great as to overcome the force of gravity, by 
which they are drawn towards the earth. 

Frederick. — Is it owing to this cause that the birds 
called kites keep themselves stationary in the air, without 
seeming to move their wings 1 

Mr. P. — The power which kites possess of soaring, as 
it is termed, in the air, depends entirely upon the slanting 
position in which they can place their bodies and their 
wings to meet the wind. They are supported, indeed, in 
exactly the same manner as the paper kite, which derives 
its name, at least, if not its invention, from these birds. 

Robert. — But the reflection of the wind against the 
paper kite, father, you said, was occasioned by the resist- 
ance of the string ; and as the bird is not tied to any thing, 
how can it offer sufficient resistance to produce the reflec- 
tion of the wind ? 

Mr. P. — That is a very proper question, Robert ; and I 
will endeavor to make you understand how the reflection is 
produced without the string, or the motion of the bird. In 
this case, as in that of the bird ascending by the velocity 
gained in its descent, the weight of the kite, or the force 
with which it is drawn to the ground, supports it in 
the air. 

Harriet. — How very odd! What, papa, is the same 
thing that is pulling the bird towards the ground, the cause 
of its being kept in the air ? 

Mr. P. — Indeed it is so, strange as it may sound. I 
will show you how this effect is produced. Suppose 
a b c [Fig. XL.] to represent the breast, tail, and wing 
of a kite soaring in an oblique position. If the bird be a. 
large one, it will weigh four pounds, and will, therefore, be 
drawn towards the earth, in the perpendicular direction 
e p, with a force equal to four pounds. The wind, blow- 



248 



FLYING. 




-Jh 



ing in the direction w e, and striking upon the oblique sur- 
face of the bird, is reflected towards e r ; the reaction is, 

consequently, from r 
Fig. XL. to e, and the real di- 

rection resulting from 
the composition of the 
two forces is from d to 
e. Now, supposing the 
actual force of the wind 
acting upon the whole 
inclining surface of 
the bird from d to e, 
(and tending to carry 
it towards g,) to be 
eight pounds ; the bird would then be acted upon by the 
gravitating force e p, with half the power (viz. four pounds) 
by which it would be impelled towards g, and the actual 
motion of the bird would be in the direction e h, which is 
the diagonal of the parallelogram e g h p, the two sides of 
which, e g and e p t represent the directions and compara- 
tive power of the two contending forces. 

Frederick, — Then the weight of the bird acts in the 
same manner as the string of the paper kite, in producing a 
reflecting power ? 

Mr. P. — It does so ; but as the weight of the bird is so 
considerable, it can only be sustained in the air in this 
manner during a brisk wind. Of course, the greater the 
force of the wind, the greater will be its power of support- 
ing and raising the kite. 

Robert. — Kites do not remain stationary in the air, I 
suppose, at any time when they are soaring ? 

Mr. P. — No; they will be always carried in the direc- 
tion of the wind, though their weight, which acts as a string 



FLYING. 249 

pulling them towards the ground, prevents them from being 
blown away rapidly, like the clouds. 

Harriet. — I wish some one would take an idea from the 
birds, to construct a machine that would make us fly too. 

Mr. P. — The attempt has been often made, but without 
success. The weight of the human body is so great, that 
we have scarcely strength enough in our arms to raise our- 
selves, even when taking hold of unyielding substances ; 
and all attempts to get a purchase, or hold, upon the air, 
strong enough to raise the body from the ground, have 
hitherto failed. The most successful attempt at flying that 
I have heard of, was that of an ingenious baronet, in York- 
shire, who contrived wings by which he could fly from tree 
to tree. 

Harriet. — How did he do that, papa? I should like, 
above all things, to see him flying. 

Mr. P. — He proceeded upon the principle of obtaining 
a horizontal motion by downward pressure, in the same 
manner as birds do when they fly through the air without 
moving their wings. He constructed wings, which he 
attached to his body ; and having ascended to the top of a 
tree, he then launched himself into the air. The wings im- 
mediately expanded, as he began to descend, and presented 
such an extended surface to the resistance of the air as to 
cause a very gradual descent; and, by inclining one of 
the wings to the ground, the flying baronet was enabled to 
give to his fall an oblique direction, so as to arrive at the 
bottom of another tree at some distance. Thus he may 
literally be said to have flown from tree to tree. 

Frederick. — Do you consider it impossible, father, for 
any machine to be invented to enable us to fly 1 

Mr. P. — Not absolutely impossible ; for if an apparatus 
could be contrived in which the muscles of the legs, as well 
as of the arms, could be made to act with advantage 



250 FLYING. 

against the air, and at the same time the descent were 
retarded by the expansion of an extensive surface, I can 
suppose it very possible for a man to support himself, and 
move through the air, provided he commenced his aerial 
excursion from a considerable height. The most probable 
means, however, by which man can hope to traverse the 
air at will, is by the aid of steam engines. 

Harriet. — Steam engines, papa! What, fly with those 
heavy things? 

Mr. P. — It is possible for steam engines to be made not 
heavier than a man, yet possessing more than ten times the 
power of one. If a proper apparatus could be invented to 
be worked against the air by such an engine, the attempt 
to fly, hitherto so unsuccessfully made, might indeed be 
carried into effect. 

Harriet. — I wish it could. I do so envy the birds on 
that account. 

Robert. — What prevents people from using balloons to 
sail with through the air ? 

Mr. P. — Because no effectual plan has been discovered 
of guiding them. They move always in the direction of 
the wind ; and though, I believe, aeronauts have succeeded, 
by the use of large wings, in altering their course in some 
small degree, yet no material deviation from the course of 
the wind can be expected to be accomplished. 

Robert. — Why cannot they use sails and a rudder, as 
on board a ship 1 

Mr. P. — You must remember, Robert, that the principal 
cause of a ship deviating from the wind is the resistance 
which the water offers to its motion sideways, and that the 
rudder acts only when it meets a resisting force. Balloons, 
on the contrary, meet with no resisting force against which 
a rudder could act with any effect, since they move nearly 
as fast as the wind. The only resisting power that could 



FLYING. 251 

be turned to account, in guiding balloons, is the resistance 
of the air to their ascent ; and that would have scarcely any 
effect against the wind. 

Harriet. — If you could invent a sailing balloon or a 
flying machine, papa, I might go to school to-morrow flying 
smoothly through the air, instead of jolting along the road 
in a carriage, till I am almost tired to death ; and how it 
would astonish them all at school ! 

Robert. — You will astonish them enough, Harriet, if 
you only tell them some of the things we have learnt since 
we came home. — I wonder what my schoolfellows will say 
when I tell them they cannot see my black hat. 

Harriet. — And mine, when I tell them that fanning 
makes the air hotter. 

Robert. — I shall have plenty of things to puzzle them 
with. — I shall tell them that cold iron is as hot as wool — 
that the wind is not cold — that snow is very hot — that coals 
do not burn — that burning is not destroying — that fire is 
hidden in cold water — that darkness is light — that water 
may boil by being cooled — that white is not white, and that 
black cannot be seen — that nothing is of a natural size — 
that a mite seen as large as a crab is not magnified — that 
birds rise in the air by being pulled down to the earth — 
that 

Harriet. — Do not forget to tell them, Robert, that you 
cannot distinguish hot from cold. 

Robert. — You will never have done with that, Harriet. 

Harriet. — Yes, I will — till we meet again at mid- 
summer. 

Mr. P. — I hope you will all, in the mean time, often 
think upon the subjects of our conversations. The knowl- 
edge you have gained will, I trust, be of use in enabling 
you to explain many other phenomena than those I have 
more particularly mentioned; and should any natural 



252 FLYING. 

occurrences present themselves that you cannot understand, 
I wish you would note them down, and when we again 
meet, I may, perhaps, be able to throw some light upon the 
matter. 

Frederick. — Thank you, father, for the very interesting 
information you have already given us. It has made me 
wish to know a great deal more on these subjects, and I 
shall be quite delighted to continue our conversations at 
midsummer. 



QUESTIONS. 



1. How are birds able to support themselves in the air? 

2. With what force must their wings strike against the air ? 

3. In what manner are large birds, that move their wings slowly, 
supported ? 

4. How is their horizontal motion produced? 

5. Birds sometimes descend rapidly through the air, and then 
ascend again, without moving their wings : what force carries them 
up ? — How do they change their direction ? 

6. How do birds change their horizontal direction ? 

7. Does the resistance of the air help to sustain birds ? — How ? — 
See Figure 39. 

8. How do the birds, called kites, keep themselves stationary in 
the air ? Explain Figure 40. 

9. Can balloons be directed through the air, "by means of sails and 
a rudder ? — Why not ? 



THE END. 



Fig. I. 



Fig. II. 




Fig. III. 




Fig. IV. 





Fig. VI. 




Fig. VII. 



I ! 



SJ 



22 



Fig. VIII. 



Fig. IX. 




ill / /.< 

m-r-r — t~t- 



mt=x- 



s^;— 



1 K<\ 



Fisr. X. 




Fig. XI. 




Fig. XII. 



Fijr. XIII. 





Fig. XIV. 



Fig. XVI. 




.Afe^ ^jg^jjj^s 




/*\ 



aft 



I / \ 




Fig. XVII. 
c 



22* 



Fig. XVIII. 




Fig. XIX. 




Fig. XX. 





Fig. XXII. 




Fig. XXIII. 




Fig. XXIV. 




Fig. XXV. 




Fig. XXVI. 



Fig. XXVII. 




Fig. XXVIII. 




Fig. XXIX. 




Fig. XXX. 






23 





Fig. XXXV. 




Fig. XXXVI. 




Fig. XXXVII. 




Fig. XXXVIII. 



Fig. XXXIX. 




Fig. XL. 



x1§ 




24 



* 



LRpFe'15 




003 644 204 9 # 



